Sliding Sync: Easier to understand timeline assertions in tests (#17511)

Added `_assertTimelineEqual(...)` because I got fed up trying to
understand the crazy diffs from the standard
`self.assertEqual(...)`/`self.assertListEqual(...)`

Before:
```
[FAIL]
Traceback (most recent call last):
  File "/home/eric/Documents/github/element/synapse/tests/rest/client/sliding_sync/test_rooms_timeline.py", line 103, in test_rooms_limited_initial_sync
    self.assertListEqual(
  File "/usr/lib/python3.12/unittest/case.py", line 1091, in assertListEqual
    self.assertSequenceEqual(list1, list2, msg, seq_type=list)
  File "/usr/lib/python3.12/unittest/case.py", line 1073, in assertSequenceEqual
    self.fail(msg)
twisted.trial.unittest.FailTest: Lists differ: ['$4QcmnzhdazSnDYcYSZCS_6-MWSzM_dN3RC7TRvW0w[95 chars]isM'] != ['$8N1XJ7e-3K_wxAanLVD3v8KQ96_B5Xj4huGkgy4N4[95 chars]nnU']

First differing element 0:
'$4QcmnzhdazSnDYcYSZCS_6-MWSzM_dN3RC7TRvW0wWA'
'$8N1XJ7e-3K_wxAanLVD3v8KQ96_B5Xj4huGkgy4N4-E'

- ['$4QcmnzhdazSnDYcYSZCS_6-MWSzM_dN3RC7TRvW0wWA',
-  '$8N1XJ7e-3K_wxAanLVD3v8KQ96_B5Xj4huGkgy4N4-E',
? ^

+ ['$8N1XJ7e-3K_wxAanLVD3v8KQ96_B5Xj4huGkgy4N4-E',
? ^

-  '$q4PRxQ_pBZkQI1keYuZPTtExQ23DqpUI3-Lxwfj_isM']
+  '$4QcmnzhdazSnDYcYSZCS_6-MWSzM_dN3RC7TRvW0wWA',
+  '$j3Xj-t2F1wH9kUHsI8X5yqS7hkdSyN2owaArfvk8nnU']
```

After:

```
[FAIL]
Traceback (most recent call last):
  File "/home/eric/Documents/github/element/synapse/tests/rest/client/sliding_sync/test_rooms_timeline.py", line 178, in test_rooms_limited_initial_sync
    self._assertTimelineEqual(
  File "/home/eric/Documents/github/element/synapse/tests/rest/client/sliding_sync/test_rooms_timeline.py", line 110, in _assertTimelineEqual
    self._assertListEqual(
  File "/home/eric/Documents/github/element/synapse/tests/rest/client/sliding_sync/test_rooms_timeline.py", line 79, in _assertListEqual
    self.fail(f"{diff_message}\n{message}")
twisted.trial.unittest.FailTest: Items must
Expected items to be in actual ('?' = missing expected items):
 [
   (10, master) $w-BoqW1PQQFU4TzVJW5OIelugxh0mY12wrfw6mbC6D4 (m.room.message) activity4
   (11, master) $sSidTZf1EOQmCVDU4mrH_1-bopMQhwcDUO2IhoemR6M (m.room.message) activity5
?  (12, master) $bgOcc3D-2QSkbk4aBxKVyOOQJGs7ZuncRJwG3cEANZg (m.room.member, @user1:test) join
 ]
Actual ('+' = found expected items):
 [
+  (11, master) $sSidTZf1EOQmCVDU4mrH_1-bopMQhwcDUO2IhoemR6M (m.room.message) activity5
+  (10, master) $w-BoqW1PQQFU4TzVJW5OIelugxh0mY12wrfw6mbC6D4 (m.room.message) activity4
   (9, master) $FmCNyc11YeFwiJ4an7_q6H0LCCjQOKd6UCr5VKeXXUw (m.room.message, None) activity3
 ]
```

2 files changed, 104 insertions, 21 deletions

diff --git a/changelog.d/17511.misc b/changelog.d/17511.misc
new file mode 100644
index 0000000000..abc7be44ba
--- /dev/null
+++ b/changelog.d/17511.misc
@@ -0,0 +1 @@
+Easier to understand `timeline` assertions in Sliding Sync tests.
diff --git a/tests/rest/client/sliding_sync/test_rooms_timeline.py b/tests/rest/client/sliding_sync/test_rooms_timeline.py
index 84a1e0d223..2e9586ca73 100644
--- a/tests/rest/client/sliding_sync/test_rooms_timeline.py
+++ b/tests/rest/client/sliding_sync/test_rooms_timeline.py
@@ -12,13 +12,14 @@
 # <https://www.gnu.org/licenses/agpl-3.0.html>.
 #
 import logging
+from typing import List, Optional
 
 from twisted.test.proto_helpers import MemoryReactor
 
 import synapse.rest.admin
 from synapse.rest.client import login, room, sync
 from synapse.server import HomeServer
-from synapse.types import StreamToken
+from synapse.types import StreamToken, StrSequence
 from synapse.util import Clock
 
 from tests.rest.client.sliding_sync.test_sliding_sync import SlidingSyncBase
@@ -42,6 +43,82 @@ class SlidingSyncRoomsTimelineTestCase(SlidingSyncBase):
         self.store = hs.get_datastores().main
         self.storage_controllers = hs.get_storage_controllers()
 
+    def _assertListEqual(
+        self,
+        actual_items: StrSequence,
+        expected_items: StrSequence,
+        message: Optional[str] = None,
+    ) -> None:
+        """
+        Like `self.assertListEqual(...)` but with an actually understandable diff message.
+        """
+
+        if actual_items == expected_items:
+            return
+
+        expected_lines: List[str] = []
+        for expected_item in expected_items:
+            is_expected_in_actual = expected_item in actual_items
+            expected_lines.append(
+                "{}  {}".format(" " if is_expected_in_actual else "?", expected_item)
+            )
+
+        actual_lines: List[str] = []
+        for actual_item in actual_items:
+            is_actual_in_expected = actual_item in expected_items
+            actual_lines.append(
+                "{}  {}".format("+" if is_actual_in_expected else " ", actual_item)
+            )
+
+        newline = "\n"
+        expected_string = f"Expected items to be in actual ('?' = missing expected items):\n [\n{newline.join(expected_lines)}\n ]"
+        actual_string = f"Actual ('+' = found expected items):\n [\n{newline.join(actual_lines)}\n ]"
+        first_message = "Items must"
+        diff_message = f"{first_message}\n{expected_string}\n{actual_string}"
+
+        self.fail(f"{diff_message}\n{message}")
+
+    def _assertTimelineEqual(
+        self,
+        *,
+        room_id: str,
+        actual_event_ids: List[str],
+        expected_event_ids: List[str],
+        message: Optional[str] = None,
+    ) -> None:
+        """
+        Like `self.assertListEqual(...)` for event IDs in a room but will give a nicer
+        output with context for what each event_id is (type, stream_ordering, content,
+        etc).
+        """
+        if actual_event_ids == expected_event_ids:
+            return
+
+        event_id_set = set(actual_event_ids + expected_event_ids)
+        events = self.get_success(self.store.get_events(event_id_set))
+
+        def event_id_to_string(event_id: str) -> str:
+            event = events.get(event_id)
+            if event:
+                state_key = event.get_state_key()
+                state_key_piece = f", {state_key}" if state_key is not None else ""
+                return (
+                    f"({event.internal_metadata.stream_ordering: >2}, {event.internal_metadata.instance_name}) "
+                    + f"{event.event_id} ({event.type}{state_key_piece}) {event.content.get('membership', '')}{event.content.get('body', '')}"
+                )
+
+            return f"{event_id} <event not found in room_id={room_id}>"
+
+        self._assertListEqual(
+            actual_items=[
+                event_id_to_string(event_id) for event_id in actual_event_ids
+            ],
+            expected_items=[
+                event_id_to_string(event_id) for event_id in expected_event_ids
+            ],
+            message=message,
+        )
+
     def test_rooms_limited_initial_sync(self) -> None:
         """
         Test that we mark `rooms` as `limited=True` when we saturate the `timeline_limit`
@@ -85,17 +162,18 @@ class SlidingSyncRoomsTimelineTestCase(SlidingSyncBase):
             response_body["rooms"][room_id1],
         )
         # Check to make sure the latest events are returned
-        self.assertEqual(
-            [
+        self._assertTimelineEqual(
+            room_id=room_id1,
+            actual_event_ids=[
                 event["event_id"]
                 for event in response_body["rooms"][room_id1]["timeline"]
             ],
-            [
+            expected_event_ids=[
                 event_response4["event_id"],
                 event_response5["event_id"],
                 user1_join_response["event_id"],
             ],
-            response_body["rooms"][room_id1]["timeline"],
+            message=str(response_body["rooms"][room_id1]["timeline"]),
         )
 
         # Check to make sure the `prev_batch` points at the right place
@@ -227,16 +305,17 @@ class SlidingSyncRoomsTimelineTestCase(SlidingSyncBase):
             + str(response_body["rooms"][room_id1]),
         )
         # Check to make sure the latest events are returned
-        self.assertEqual(
-            [
+        self._assertTimelineEqual(
+            room_id=room_id1,
+            actual_event_ids=[
                 event["event_id"]
                 for event in response_body["rooms"][room_id1]["timeline"]
             ],
-            [
+            expected_event_ids=[
                 event_response2["event_id"],
                 event_response3["event_id"],
             ],
-            response_body["rooms"][room_id1]["timeline"],
+            message=str(response_body["rooms"][room_id1]["timeline"]),
         )
 
         # All events are "live"
@@ -303,18 +382,19 @@ class SlidingSyncRoomsTimelineTestCase(SlidingSyncBase):
             + str(response_body["rooms"][room_id1]),
         )
         # Check to make sure that the "live" and historical events are returned
-        self.assertEqual(
-            [
+        self._assertTimelineEqual(
+            room_id=room_id1,
+            actual_event_ids=[
                 event["event_id"]
                 for event in response_body["rooms"][room_id1]["timeline"]
             ],
-            [
+            expected_event_ids=[
                 event_response2["event_id"],
                 user1_join_response["event_id"],
                 event_response3["event_id"],
                 event_response4["event_id"],
             ],
-            response_body["rooms"][room_id1]["timeline"],
+            message=str(response_body["rooms"][room_id1]["timeline"]),
         )
 
         # Only events after the `from_token` are "live" (join, event3, event4)
@@ -361,17 +441,18 @@ class SlidingSyncRoomsTimelineTestCase(SlidingSyncBase):
         response_body, _ = self.do_sync(sync_body, tok=user1_tok)
 
         # We should see events before the ban but not after
-        self.assertEqual(
-            [
+        self._assertTimelineEqual(
+            room_id=room_id1,
+            actual_event_ids=[
                 event["event_id"]
                 for event in response_body["rooms"][room_id1]["timeline"]
             ],
-            [
+            expected_event_ids=[
                 event_response3["event_id"],
                 event_response4["event_id"],
                 user1_ban_response["event_id"],
             ],
-            response_body["rooms"][room_id1]["timeline"],
+            message=str(response_body["rooms"][room_id1]["timeline"]),
         )
         # No "live" events in an initial sync (no `from_token` to define the "live"
         # range)
@@ -428,17 +509,18 @@ class SlidingSyncRoomsTimelineTestCase(SlidingSyncBase):
         response_body, _ = self.do_sync(sync_body, since=from_token, tok=user1_tok)
 
         # We should see events before the ban but not after
-        self.assertEqual(
-            [
+        self._assertTimelineEqual(
+            room_id=room_id1,
+            actual_event_ids=[
                 event["event_id"]
                 for event in response_body["rooms"][room_id1]["timeline"]
             ],
-            [
+            expected_event_ids=[
                 event_response3["event_id"],
                 event_response4["event_id"],
                 user1_ban_response["event_id"],
             ],
-            response_body["rooms"][room_id1]["timeline"],
+            message=str(response_body["rooms"][room_id1]["timeline"]),
         )
         # All live events in the incremental sync
         self.assertEqual(