Add separate support for xUnit v3

[JohSand]

Add simple tests

Implement xUnit testcases for xUnit.v3

[JohSand]

fixes xUnit part of #690

The changes in xUnit are breaking, and they choose to publish a package with a different name (xUnit.v3). I believe the most reasonable way for this project is to have different nugets for FsCheck.xUnit and FsCheck.xUnit.v3.

In this I attempt the simplest possible translation of the previous implementation to the new abstractions offered in v3. It is mostly copied directly from the v2 implementation.

It suffers a bit from targeting netstandard2.0 (no task computation expression support), but I feel it is reasonable.

[kurtschelfthout]

This looks great - thanks

It suffers a bit from targeting netstandard2.0 (no task computation expression support), but I feel it is reasonable.

Fwiw fine to target something more recent. E.g. FsCheck.NUnit now targets net6.0 since NUnit's latest major version update.

I'll leave this here for a bit in case you want to change that, but also happy to merge as-is.

[JohSand]

Thanks for the review. I believe we can merge as-is. But it occurs to me that I made no changes to build.fsx, so no new Nuget will be published.