Sliding Sync: Move filters tests to rest layer (#17703)

Move filters tests to rest layer in order to test the new (with sliding
sync tables) and fallback paths that Sliding Sync can use.

Also found a bug in the new path because it's not being tested which is
also fixed in this PR. We now take into account `has_known_state` when
filtering.

Spawning from
https://github.com/element-hq/synapse/pull/17662#discussion_r1755574791.
This should have been done when we started using the new sliding sync
tables in https://github.com/element-hq/synapse/pull/17630