The code here a proof of concept, not a ready to use tool. But it works. It
converts a few types of NUnit Assert expressions to their equivalents in
xUnit. It also cleans up a couple of other small things, like [Test] to
[Fact] conversion or removal of [TestFixture].
I wrote about this project in detail here, here and here.
The library is released under the MIT license. See LICENSE for details.