Fix a few more multiplayer test timeouts

These can be tested by adding a `Task.Delay(3000);` at the end of the
`MultiplayerClient.JoinRoom` task. The reason is typically that
`Client.Room` becomes not-null but the join task still hasn't completed
yet, so e.g. the ready button is still disabled.

osu.Game.Tests/NonVisual/Multiplayer/StatefulMultiplayerClientTest.cs

@@ -72,7 +72,7 @@ namespace osu.Game.Tests.NonVisual.Multiplayer
                 RoomManager.CreateRoom(newRoom);
             });
 
-            AddUntilStep("wait for room join", () => Client.Room != null);
+            AddUntilStep("wait for room join", () => RoomJoined);
             checkPlayingUserCount(1);
         }