Unshelve Xunit2Parallel;angocke and:

1. Run .nuget\NuGetRestore.ps1
2. Build the Roslyn solution.
3. Run .\packages\xunit.runners.2.0.0-rc1-build2826\tools\xunit.console.exe .\Binaries\Debug\Roslyn.Services.UnitTests.dll -parallel assemblies -noshadow

This should deadlock (you can investigate by attaching to the xunit process). I suspect the problem is StopTheThreadPoolContext.