Add very basic visual tests for missing beatmap notification

The full-stack test using the whole 9 `OsuGameTest` yards is unusable
for rapid development. I don't really get how you could ever design
anything using it without tossing your computer out the window.

osu.Game.Tests/Visual/UserInterface/TestSceneMissingBeatmapNotification.cs

@@ -0,0 +1,34 @@
+// 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 System.IO;
+using System.Linq;
+using NUnit.Framework;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Game.Database;
+using osu.Game.Overlays;
+
+namespace osu.Game.Tests.Visual.UserInterface
+{
+    [TestFixture]
+    public partial class TestSceneMissingBeatmapNotification : OsuTestScene
+    {
+        [Cached]
+        private OverlayColourProvider colourProvider = new OverlayColourProvider(OverlayColourScheme.Purple);
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            Child = new Container
+            {
+                Width = 280,
+                AutoSizeAxes = Axes.Y,
+                Anchor = Anchor.Centre,
+                Origin = Anchor.Centre,
+                Child = new MissingBeatmapNotification(CreateAPIBeatmapSet(Ruleset.Value).Beatmaps.First(), new MemoryStream(), "deadbeef")
+            };
+        }
+    }
+}