Update unit tests to use new default theme URLs.

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/BasicTests.cs

@@ -528,7 +528,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
                 var xaml = @"
 <Styles xmlns='https://github.com/avaloniaui'
         xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
-    <StyleInclude Source='resm:Avalonia.Themes.Default.ContextMenu.xaml?assembly=Avalonia.Themes.Default'/>
+    <StyleInclude Source='avares://Avalonia.Themes.Default/Controls/ContextMenu.xaml'/>
 </Styles>";
 
                 var styles = AvaloniaRuntimeXamlLoader.Parse<Styles>(xaml);

tests/Avalonia.UnitTests/TestServices.cs

@@ -168,7 +168,7 @@ namespace Avalonia.UnitTests
             };
 
             var baseLight = (IStyle)AvaloniaXamlLoader.Load(
-                new Uri("resm:Avalonia.Themes.Default.Accents.BaseLight.xaml?assembly=Avalonia.Themes.Default"));
+                new Uri("avares://Avalonia.Themes.Default/Accents/BaseLight.xaml"));
             result.Add(baseLight);
 
             return result;

tests/Avalonia.UnitTests/UnitTestApplication.cs

@@ -10,6 +10,7 @@ using System.Reactive.Disposables;
 using System.Reactive.Concurrency;
 using Avalonia.Input.Platform;
 using Avalonia.Animation;
+using Avalonia.Shared.PlatformSupport;
 
 namespace Avalonia.UnitTests
 {
@@ -29,6 +30,11 @@ namespace Avalonia.UnitTests
             RegisterServices();
         }
 
+        static UnitTestApplication()
+        {
+            AssetLoader.RegisterResUriParsers();
+        }
+
         public static new UnitTestApplication Current => (UnitTestApplication)Application.Current;
 
         public TestServices Services => _services;