Add tests for or prevent non-member events in `state_events_at_start` in MSC2716 batch sending

[tulir]

Synapse 1.53 broke this branch of batch sending: https://github.com/matrix-org/synapse/blob/v1.53.0/synapse/handlers/room_batch.py#L232-L253

#12083 seems to somehow fix it, but the feature should get some tests or be removed entirely.

[MadLittleMods]

Closing as #12329 was merged to fix this use case in Synapse and Complement tests added in matrix-org/complement#354