test: Adjust test for XamlRoot

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml/Given_UIElement.cs

@@ -220,8 +220,8 @@ namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml
 			{
 				// Ttrick - GetVisualTreeParent's return type is different
 				// on each platform, so we use var to get the correct type implicitly
-				var current = Windows.UI.Xaml.Window.Current.Content?.GetVisualTreeParent();
-				current = Windows.UI.Xaml.Window.Current.Content;
+				var current = TestServices.WindowHelper.XamlRoot.Content?.GetVisualTreeParent();
+				current = TestServices.WindowHelper.XamlRoot.Content;
 				var parent = current?.GetVisualTreeParent();
 				while (parent != null)
 				{