Add Flaky UI Test category (#23432)

Author: jfversluis

src/Controls/tests/TestCases.Shared.Tests/UITestCategories.cs

@@ -28,6 +28,7 @@ internal static class UITestCategories
 		public const string Layout = "Layout";
 		public const string ListView = "ListView";
 		public const string LifeCycle = "Lifecycle";
+		public const string Flaky = "Flaky";
 		public const string FlyoutPage = "FlyoutPage";
 		public const string Picker = "Picker";
 		public const string ProgressBar = "ProgressBar";