Add tests to verify that we can resolve member avatar/displayname state for historical messages (MSC2716) (#206)

Part of testing https://gitlab.com/gitterHQ/webapp/-/merge_requests/2229#note_654645018

Tighten up assertions by re-using `validateBatchSendRes` everywhere we can which asserts things appear in order and state exists if we want.

Author: MadLittleMods