This will allow test setup and teardown to expect PDUs and EDUs
independently of tests. In a future commit, test teardown will leave
rooms as part of cleanup and expect to see a corresponding leave event.

Make the homeserver under test leave test rooms at the end of each test,
so that it stops trying to contact other homeservers in the room.

Signed-off-by: Sean Quah <seanq@matrix.org>