test: stabilize When_RepeatForever_ShouldLoop

src/Uno.UI.RuntimeTests/Tests/Windows_UI_Xaml_Media_Animation/Given_DoubleAnimation.cs

-using System;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
@@ -121,10 +121,20 @@ namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Media_Animation
 		[RunsOnUIThread]
 		public async Task When_RepeatForever_ShouldLoop()
 		{
+			// On CI, the measurement at 100ms seem to be too unreliable on Android & MacOS.
+			// Stretch the test by 5x greatly improve the stability. When testing locally, we can used 1x to save time (5s vs 25s).
+			const int TimeResolutionScaling =
+#if !DEBUG && (__ANDROID__ || __MACOS__)
+				5;
+#else
+				1;
+#endif
+
 			var target = new Windows.UI.Xaml.Shapes.Rectangle
 			{
 				Stretch = Stretch.Fill,
 				Fill = new SolidColorBrush(Colors.SkyBlue),
+				Width = 50,
 				Height = 50,
 			};
 			WindowHelper.WindowContent = target;
@@ -137,19 +147,16 @@ namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Media_Animation
 				From = 0,
 				To = 50,
 				RepeatBehavior = RepeatBehavior.Forever,
-				Duration = TimeSpan.FromMilliseconds(500),
-			};
-			Storyboard.SetTarget(animation, target);
-			Storyboard.SetTargetProperty(animation, nameof(Rectangle.Width));
-
-			var storyboard = new Storyboard { Children = { animation } };
-			storyboard.Begin();
+				Duration = TimeSpan.FromMilliseconds(500 * TimeResolutionScaling),
+			}.BindTo(target, nameof(Rectangle.Width));
+			animation.Begin();
 
+			// In an ideal world, the measurements would be [0 or 50,10,20,30,40] repeated 10 times.
 			var list = new List<double>();
 			for (int i = 0; i < 50; i++)
 			{
-				list.Add(target.Width);
-				await Task.Delay(100);
+				list.Add(NanToZero(target.Width));
+				await Task.Delay(100 * TimeResolutionScaling);
 			}
 
 			var delta = list.Zip(list.Skip(1), (a, b) => b - a).ToArray();
@@ -160,10 +167,20 @@ namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml_Media_Animation
 				.ToArray();
 			var incrementSizes = drops.Zip(drops.Skip(1), (a, b) => b - a - 1).ToArray(); // -1 to exclude the drop itself
 
+			var context = new StringBuilder()
+				.AppendLine("list: " + string.Join(", ", list.Select(x => x.ToString("0.#"))))
+				.AppendLine("delta: " + string.Join(", ", delta.Select(x => x.ToString("+0.#;-0.#;0"))))
+				.AppendLine("averageIncrement: " + averageIncrement)
+				.AppendLine("drops: " + string.Join(", ", drops.Select(x => x.ToString("0.#"))))
+				.AppendLine("incrementSizes: " + string.Join(", ", incrementSizes.Select(x => x.ToString("0.#"))))
+				.ToString();
+
 			// This 500ms animation is expected to climb from 0 to 50, reset to 0 instantly, and repeat forever.
 			// Given that we are taking 5measurements per cycle, we can expect the followings:
-			Assert.AreEqual(10d, averageIncrement, 1.5, "an rough average of increment (exluding the drop) of 10 (+-15% error margin)");
-			Assert.IsTrue(incrementSizes.Count(x => x > 3) > 8, $"at least 10 (-2 error margin: might miss first and/or last) sets of continuous increments that size of 4 (+-1 error margin: sliding slot): {string.Join(",", incrementSizes)}");
+			Assert.AreEqual(10d, averageIncrement, 2.5, $"Expected an rough average of increment (excluding the drop) of 10 (+-25% error margin).\n" + context);
+			Assert.IsTrue(incrementSizes.Count(x => x >= 3) >= 8, $"Expected at least 10sets (-2 error margin: might miss first and/or last) of continuous increments in size of 4 (+-1 error margin: sliding slot).\n" + context);
+
+			double NanToZero(double value) => double.IsNaN(value) ? 0 : value;
 		}
 
 		[TestMethod]