Add xUnit support

[hatzhang]

So this can be used like those testcontainers.

[mariotoffia]

Thanks @hatzhang for this great idea. I'll have a check and see when I get some time to create a new nuget package for this.

[mariotoffia]

Hi @hatzhang, @jgozner has fixed this and it's about to be built in master - if everything builds OK it will be present in NuGet in about 30 minutes or so.

Cheers,
Mario :)