Fix TestCaseButtonSystem not working

Was missing an osu! logo

osu.Game.Tests/Visual/TestCaseButtonSystem.cs

@@ -0,0 +1,33 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Colour;
+using osu.Framework.Graphics.Shapes;
+using osu.Game.Screens.Menu;
+using OpenTK.Graphics;
+
+namespace osu.Game.Tests.Visual
+{
+    internal class TestCaseButtonSystem : OsuTestCase
+    {
+        public TestCaseButtonSystem()
+        {
+            OsuLogo logo;
+            ButtonSystem buttons;
+
+            Children = new Drawable[]
+            {
+                new Box
+                {
+                    Colour = ColourInfo.GradientVertical(Color4.Gray, Color4.WhiteSmoke),
+                    RelativeSizeAxes = Axes.Both,
+                },
+                buttons = new ButtonSystem(),
+                logo = new OsuLogo()
+            };
+
+            buttons.SetOsuLogo(logo);
+        }
+    }
+}