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
 ]
```

1 files changed, 1 insertions, 0 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.