fix designer tests.

tests/Avalonia.DesignerSupport.Tests/DesignerSupportTests.cs

@@ -28,12 +28,12 @@ namespace Avalonia.DesignerSupport.Tests
 
         [SkippableTheory,
          InlineData(
-            @"..\..\..\..\..\tests/Avalonia.DesignerSupport.TestApp/bin/$BUILD/netcoreapp2.0/",
+            @"..\..\..\..\..\tests/Avalonia.DesignerSupport.TestApp/bin/$BUILD/netcoreapp3.1/",
             "Avalonia.DesignerSupport.TestApp",
             "Avalonia.DesignerSupport.TestApp.dll",
             @"..\..\..\..\..\tests\Avalonia.DesignerSupport.TestApp\MainWindow.xaml"),
          InlineData(
-            @"..\..\..\..\..\samples\ControlCatalog.NetCore\bin\$BUILD\netcoreapp2.0\",
+            @"..\..\..\..\..\samples\ControlCatalog.NetCore\bin\$BUILD\netcoreapp3.1\",
             "ControlCatalog.NetCore",
             "ControlCatalog.dll",
             @"..\..\..\..\..\samples\ControlCatalog\MainWindow.xaml")]