Custom aspnet xunit extensions should be converged with XUnitExtensions in arcade

[ViktorHofer]

I just noticed that aspnetcore has xunitextensions for various use-cases: https://github.com/aspnet/Extensions/tree/master/src/TestingUtils/Microsoft.AspNetCore.Testing/src/xunit. We do have the same in corefx and partner repositories: https://github.com/dotnet/arcade/tree/master/src/Microsoft.DotNet.XUnitExtensions

We should converge both and remove the duplication here.