Convert Sliding Sync tests to use higher-level `compute_interested_rooms` (#18399)

Spawning from
https://github.com/element-hq/synapse/pull/18375#discussion_r2071768635,

This updates some sliding sync tests to use a higher level function in
order to move test coverage to cover both fallback & new tables.
Important when https://github.com/element-hq/synapse/pull/18375 is
merged.

In other words, adjust tests to target `compute_interested_room(...)`
(relevant to both new and fallback path) instead of the lower level
`get_room_membership_for_user_at_to_token(...)` that only applies to the
fallback path.

### Dev notes

```
SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.handlers.test_sliding_sync.ComputeInterestedRoomsTestCase_new
```

```
SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.rest.client.sliding_sync
```

```
SYNAPSE_POSTGRES=1 SYNAPSE_POSTGRES_USER=postgres SYNAPSE_TEST_LOG_LEVEL=INFO poetry run trial tests.handlers.test_sliding_sync.ComputeInterestedRoomsTestCase_new.test_display_name_changes_leave_after_token_range
```

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---------

Co-authored-by: Eric Eastwood <erice@element.io>

1 files changed, 2 insertions, 0 deletions

diff --git a/synapse/storage/databases/main/stream.py b/synapse/storage/databases/main/stream.py

index 00e5208674..c52389b8a9 100644
--- a/synapse/storage/databases/main/stream.py
+++ b/synapse/storage/databases/main/stream.py
@@ -453,6 +453,8 @@ def _filter_results_by_stream(
     stream_ordering falls between the two tokens (taking a None
     token to mean unbounded).
 
+    The token range is defined by > `lower_token` and <= `upper_token`.
+
     Used to filter results from fetching events in the DB against the given
     tokens. This is necessary to handle the case where the tokens include
     position maps, which we handle by fetching more than necessary from the DB