[wix-devs] Missing Package Reference

[Ron Martin]

Upon updating my Core repo, Visual Studio showed four IntelliSense 
warnings. I resolved the warnings using the NuGet Package Manager. This 
produced the following diff for CompileCoreTestExtensionWixlib.csproj:

<ItemGroup>
  <PackageReference Include="xunit.analyzers" Version="0.10.0" />
<ItemGroup>

This change corrects the problem. I doubt that we could or should 
provide a regression test for this type of error, but I leave that for 
discussion. If you want to create an issue without a failing test, I can 
submit a pull request or you could handle it yourself.

Ron Martin