I guess the changes that involved `MultiplayerTestScene` having a test user lookup cache caused this test case to false-pass silently. Added an explicit assert which ensures the added user indeed has a null `User` value.
TestMultiplayerClient
Not sure this is the best place to do so... I can foresee this class getting much larger than we want it to.
MatchRuleset
Fix client potentially not leaving a room on exiting multiplayer