Do not display window flash test scene on mobile

As it does nothing there.

osu.Framework.Tests/Visual/Platform/TestSceneWindowFlash.cs

@@ -1,6 +1,7 @@
 // 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.Runtime.Versioning;
 using NUnit.Framework;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
@@ -12,6 +13,9 @@ using osu.Framework.Platform;
 namespace osu.Framework.Tests.Visual.Platform
 {
     [Ignore("This test cannot run in headless mode (a window instance is required).")]
+    [SupportedOSPlatform("windows")]
+    [SupportedOSPlatform("linux")]
+    [SupportedOSPlatform("macos")]
     public partial class TestSceneWindowFlash : FrameworkTestScene
     {
         private IBindable<bool> isActive = null!;