Improve implementation of TestSpectatorClient

There was a lot of weirdness here, such as storing the playing users,
clearing the playing users from test scenes (!!), and storing the users
being wathed.
This was all a thing because the previous implementation overrode the
base method implementations, which is no longer a thing.

osu.Game.Tests/Visual/Multiplayer/TestSceneMultiSpectatorLeaderboard.cs

@@ -59,7 +59,7 @@ namespace osu.Game.Tests.Visual.Multiplayer
 
                 foreach (var (userId, clock) in clocks)
                 {
-                    spectatorClient.EndPlay(userId, 0);
+                    spectatorClient.EndPlay(userId);
                     clock.CurrentTime = 0;
                 }
             });