this is never set to anything other than "test", and is a source of unnecessary
boilerplate.

The expected use case is to suppress MAU limiting on small instances

The `expire_access_token` didn't do what it sounded like it should do. What it
actually did was make Synapse enforce the 'time' caveat on macaroons used as
access tokens, but since our access token macaroons never contained such a
caveat, it was always a no-op.

(The code to add 'time' caveats was removed back in v0.18.5, in #1656)

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