netstandard support for the other (xunit is a big one) FsUnit libraries

[NinoFloris]

It seems there is no FsUnit.Xunit netstandard support yet although the main package already got it in march. Would be nice to see this for the other packages as well

[sergey-tihon]

We did initial support in 3.0.0 and more accurate dependencies in 3.1.0-beta1.
Please try the latest pre-release package and open another issue if you have any kind of issue with it.