Don't make TestBackend.rooms be a property either

See 8c56fb951b60b531cd20d8261bd55f5c9a7b83a5 for additional info.
BotPlugin.rooms shouldn't be a property either.

See 8c56fb951b60b531cd20d8261bd55f5c9a7b83a5 for additional info
Testbackend: Let rooms() return only joined rooms
Add !room list command