Fix tests failing intermittently

This was due to this code happening in UpdateAfterChildren(), after the
GCC has processed one frame. During this time, the clock could have
advanced an arbitrary amount.

The cause of this is the removal of the Task.Run() to set the clock in
Restart() (now called Reset()) which changed the timing, so it only worked before due to pure
luck.

osu.Game.Tests/Visual/Gameplay/TestSceneLeadIn.cs

@@ -130,9 +130,9 @@ namespace osu.Game.Tests.Visual.Gameplay
 
             public double GameplayClockTime => GameplayClockContainer.GameplayClock.CurrentTime;
 
-            protected override void UpdateAfterChildren()
+            protected override void Update()
             {
-                base.UpdateAfterChildren();
+                base.Update();
 
                 if (!FirstFrameClockTime.HasValue)
                 {