Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)

Make sure that we have a `server_notices_mxid` set, given that we are relying
on it.

This test stubbed out some stuff in a very weird way. I have no idea why. It broke.

This manifested in synapse repeatedly setting the tag for the room

This was due to a bug where we mutated a cached event's contents