Add tests for downgrading a v12 room to one that doesn't support MSC4289

[anoadragon453]

Adds some tests for downgrading a v12 room. Also cleans up some existing tests and fixes a typo.

⚠️ MSC4289 does not specify what power level to set the room's creator (and additional_creators) to in the new room when downgrading to a room that does not support MSC4289.

The general consensus seems to be setting their power level(s) to the minimum needed to send m.room.power_levels in the room - aka being able to do anything you want in the room. Synapse implements this as taking the max of all fields under events, and state_default (which would apply to m.room.power_levels if it wasn't listed under events).

This PR would effectively be requiring the above of homeservers without the spec having yet agreed on it yet. We should hold off on merging this PR until that is decided.

Recommended to review commit-by-commit.

[anoadragon453]

@matthias:ahouansou.cz mentioned in an internal room that it'd be good to also take all user power levels into account when calculating the creator's power level in the new room. Otherwise, you won't be able to downgrade other users' power levels, whereas you could in the old room as the creator.

They also proposed just setting the creator's power level to Int::MAX. While this would look odd in clients, clients could "mask" this by additionally adding the label "Creator" (or whatever) to users with PL Int::Max in room versions that don't support MSC4289.

In addition, this would be much easier for homeservers to implement (and thus not introduce bugs) than taking the max of a bunch of different fields.