chore(SkiaSharpTest): Migrating WpfHost in SkiaSharpTest

Fix the running error as:

> If you are seeing this, make sure to follow the "Migrating WpfHost" section of Migrating from previous releases article in the Uno Platform documentation at https://aka.platform.uno/uno5-wpfhost-migration. WpfHost is used at the application level instead of window level starting Uno Platform 5.0.

Author: lindexi

UI/SkiaSharpTest/SkiaSharpTest/SkiaSharpTest.Skia.WPF/App.xaml

@@ -1,8 +1,7 @@
 <Application x:Class="SkiaSharpTest.WPF.App"
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
-             xmlns:local="clr-namespace:SkiaSharpTest.WPF"
-             StartupUri="MainWindow.xaml">
+             xmlns:local="clr-namespace:SkiaSharpTest.WPF">
     <Application.Resources>
 
     </Application.Resources>

UI/SkiaSharpTest/SkiaSharpTest/SkiaSharpTest.Skia.WPF/App.xaml.cs

@@ -6,12 +6,19 @@
 using System.Threading.Tasks;
 using System.Windows;
 
+using Uno.UI.Runtime.Skia.Wpf;
+
 namespace SkiaSharpTest.WPF
 {
     /// <summary>
     /// Interaction logic for App.xaml
     /// </summary>
     public partial class App : Application
     {
+        public App()
+        {
+            var host = new WpfHost(Dispatcher, () => new SkiaSharpTest.App());
+            host.Run();
+        }
     }
 }