Fix GC Race condition with tests (#27652)

src/Controls/tests/Core.UnitTests/TestClasses/TestWindow.cs

@@ -2,13 +2,14 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.CompilerServices;
-using System.Text;
-using System.Threading.Tasks;
 
 namespace Microsoft.Maui.Controls.Core.UnitTests
 {
 	public class TestWindow : Window
 	{
+		// Because the relationship from Window => Application is a weakreference
+		// we need to retain a reference to the Application so it doesn't get GC'd
+		TestApp _app;
 		public TestWindow()
 		{
 
@@ -24,12 +25,10 @@ protected override void OnPropertyChanged([CallerMemberName] string propertyName
 			if (propertyName == PageProperty.PropertyName &&
 				Parent == null)
 			{
-				var app = new TestApp(this);
-				_ = (app as IApplication).CreateWindow(null);
+				_app = new TestApp(this);
+				_ = (_app as IApplication).CreateWindow(null);
 			}
 		}
-
-
 	}
 
 	public static class TestWindowExtensions