Update to Avalonia v11.3.9 and Implement ResourceDictionary DSL

src/Avalonia.FuncUI.UnitTests/Assets/TestResources1.xaml

@@ -0,0 +1,6 @@
+<ResourceDictionary xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <SolidColorBrush x:Key='BrushA' Color='Red' />
+    <SolidColorBrush x:Key='BrushB' Color='Green' />
+    <SolidColorBrush x:Key='BrushC' Color='Blue' />
+</ResourceDictionary>

src/Avalonia.FuncUI.UnitTests/Assets/TestResources2.xaml

@@ -0,0 +1,6 @@
+<ResourceDictionary xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <SolidColorBrush x:Key='BrushA' Color='White' />
+    <SolidColorBrush x:Key='BrushB' Color='Black' />
+    <SolidColorBrush x:Key='BrushC' Color='Gray' />
+</ResourceDictionary>

src/Avalonia.FuncUI.UnitTests/Avalonia.FuncUI.UnitTests.fsproj

@@ -11,17 +11,25 @@
     <Optimize>false</Optimize>
   </PropertyGroup>
 
+  <ItemGroup>
+    <AvaloniaResource Include="**\*.xaml" />
+  </ItemGroup>
+
   <ItemGroup>
     <Compile Include="LibTests.fs" />
     <Compile Include="VirtualDom\VirtualDom.ModuleTests.fs" />
     <Compile Include="VirtualDom\VirtualDom.DifferTests.fs" />
     <Compile Include="VirtualDom\VirtualDom.PatcherTests.fs" />
+    <Compile Include="DSL\TestHelpers.fs" />
+    <Compile Include="DSL\Base\ResourceDictionaryTests.fs" />
     <Compile Include="DSL\Base\StyledElementTests.fs" />
     <Compile Include="State.fs" />
     <Compile Include="Program.fs" />
   </ItemGroup>
 
   <ItemGroup>
+    <PackageReference Include="Avalonia.Headless" Version="$(AvaloniaVersion)" />
+    <PackageReference Include="Avalonia.Headless.Xunit" Version="$(AvaloniaVersion)" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
     <PackageReference Include="System.Reactive" Version="6.0.1" />
     <PackageReference Include="xunit" Version="2.9.0" />