Add back visual tests and add easing to alpha fade.

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

@@ -0,0 +1,31 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using NUnit.Framework;
+using osu.Game.Screens.Play.HUD;
+
+namespace osu.Game.Tests.Visual.Gameplay
+{
+    public class TestSceneFailingLayer : OsuTestScene
+    {
+        private readonly FailingLayer layer;
+
+        public TestSceneFailingLayer()
+        {
+            Child = layer = new FailingLayer();
+        }
+
+        [Test]
+        public void TestLayerFading()
+        {
+            AddSliderStep("current health", 0.0, 1.0, 1.0, val =>
+            {
+                layer.Current.Value = val;
+            });
+
+            AddStep("set health to 0.10", () => layer.Current.Value = 0.10);
+            AddWaitStep("wait for fade to finish", 5);
+            AddStep("set health to 1", () => layer.Current.Value = 1f);
+        }
+    }
+}