Test relations (MSC2675) and threads (MSC3440, et al)

[clokep]

Synapse has a variety of unit tests for MSC2675 / MSC3440 (and MSC2676 / MSC2677, but those are still unstable). Some of those would be good to be complement tests instead.

See https://github.com/matrix-org/synapse/blob/4df10d32148ae29f792afc68ff774bcbd1915cea/tests/rest/client/test_relations.py

[clokep]

I think the main things left to test here is:

• Ignoring users
• Redacting events