fix: Make ImageStretch_None wait for images to actually load

Images displayed on the ImageStretch_None page are coming from the uno-assets.platform.uno site, and may not have loaded yet when the sample runs

src/SamplesApp/SamplesApp.UITests/Windows_UI_Xaml_Controls/ImageTests/UnoSamples_Tests.Image.cs

@@ -65,8 +65,7 @@ public void ImageStretch_None()
 			void HasValidSize(string name)
 			{
 				var rect = _app.Query(q => q.All().Marked(name)).First().Rect;
-				Assert.That(rect.Width != 0);
-				Assert.That(rect.Height != 0);
+				_app.WaitFor(() => rect.Width != 0 && rect.Height != 0, timeout: TimeSpan.FromSeconds(2));
 			}
 
 			HasValidSize("image01");