fix designer tests.

samples/ControlCatalog.NetCore/ControlCatalog.NetCore.csproj

@@ -5,6 +5,7 @@
     <TargetFramework>net6.0</TargetFramework>
     <TargetLatestRuntimePatch>true</TargetLatestRuntimePatch>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <RuntimeFrameworkVersion>6.0.9</RuntimeFrameworkVersion>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(RunNativeAotCompilation)' == 'true'">

src/tools/Avalonia.Designer.HostApp/Avalonia.Designer.HostApp.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks>net461;netcoreapp2.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.1</TargetFrameworks>
   </PropertyGroup>
   <!--Disable Net Perf. analyzer for submodule to avoid commit issue -->
   <PropertyGroup>

tests/Avalonia.DesignerSupport.Tests/DesignerSupportTests.cs

@@ -19,7 +19,7 @@ namespace Avalonia.DesignerSupport.Tests
 {
     public class DesignerSupportTests
     {
-        private const string DesignerAppPath = "../../../../../src/tools/Avalonia.Designer.HostApp/bin/$BUILD/netcoreapp2.0/Avalonia.Designer.HostApp.dll";
+        private const string DesignerAppPath = "../../../../../src/tools/Avalonia.Designer.HostApp/bin/$BUILD/netstandard2.1/Avalonia.Designer.HostApp.dll";
         private readonly Xunit.Abstractions.ITestOutputHelper outputHelper;
 
         public DesignerSupportTests(Xunit.Abstractions.ITestOutputHelper outputHelper)