Log when events are (unexpectedly) filtered out of responses in tests (#14213)
See https://github.com/matrix-org/synapse/pull/14095#discussion_r990335492
This is useful because when see that a relevant event is an `outlier` or `soft-failed`, then that's a good unexpected indicator explaining why it's not showing up. `filter_events_for_client` is used in `/sync`, `/messages`, `/context` which are all common end-to-end assertion touch points (also notifications, relations).
"_check_client_allowed_to_see_event(event=%s): Filtered out event because the user can't see the event because of their membership, membership_result.allowed=%s membership_result.joined=%s",