Fix test failure in TestSceneMultiplayerGameplayLeaderboard

The transfer of users was not accounting for the fact that the
`StartPlay` calls are now scheduled and not necessarily run in time.

osu.Game/Tests/Visual/Spectator/TestSpectatorClient.cs

@@ -3,6 +3,7 @@
 
 #nullable enable
 
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -53,6 +54,8 @@ namespace osu.Game.Tests.Visual.Spectator
             });
         }
 
+        public new void Schedule(Action action) => base.Schedule(action);
+
         /// <summary>
         /// Sends frames for an arbitrary user.
         /// </summary>