Add axplicit packagerefs in test project

They are no longer added transitively

Tests/MonoDevelop.Xml.Tests.csproj

@@ -33,4 +33,10 @@
     <PackageReference Include="NUnit" Version="3.12.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.13.0" />
   </ItemGroup>
+ <ItemGroup>
+    <PackageReference Include="Microsoft.VisualStudio.ImageCatalog" Version="16.9.31023.347" />
+    <PackageReference Include="Microsoft.VisualStudio.Language.Intellisense" Version="16.9.227" />
+    <PackageReference Include="Microsoft.VisualStudio.Language.StandardClassification" Version="16.9.227" />
+    <PackageReference Include="Microsoft.VisualStudio.Threading" Version="16.9.60" />
+ </ItemGroup>
 </Project>