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>

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