Fix missing using statements in multiple test scenes

Author: peppy

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

@@ -8,6 +8,7 @@
 using osu.Framework.Allocation;
 using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
 using osu.Framework.Testing;
 using osu.Framework.Timing;
 using osu.Game.Configuration;

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

@@ -10,6 +10,8 @@
 using osu.Framework.Allocation;
 using osu.Framework.Extensions.IEnumerableExtensions;
 using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Testing;
 using osu.Framework.Timing;
 using osu.Game.Rulesets;
 using osu.Game.Rulesets.Mods;