Upgrade to `dotnet xunit`

[heynickc]

dotnet xunit is out and allows passing -teamcity as an argument to force TeamCity formatting. This wasn't available in dotnet test so there's been the chance that we'd get false positives in our Mono Builds in TeamCity.

dotnet xunit installation notes: https://xunit.github.io/docs/getting-started-dotnet-core.html