There are some very weird behaviour here. I've been tinkering around with disabling test specifications, and I got it to run successfully when 1 out of 3 test specifications are disabled. To me this makes no sense.
Turning off parallel test execution is not really a solution in this case since parallel test execution works. Locally I run the tests in parallel within the test assembly, both with the xunit console runner and ncrunch, and it works fine.
This repo is a testing framework. Being able to run tests in parallel is quite important.
Right, it works OK on AppVeyor too if I spin up "Premium" build VM (which is in beta testing now) with 4 cores for this build. Standard build VMs have 2 CPU cores. I am quite sure that your development VM has more than 2 cores too. Also it is pretty possible (though I can be mistaken) that if you run the same tests on some another VM outside of AppVeyor with 1 or 2 cores, you will see the same issue.
Definitely it is a bug somewhere, not necessary in AppVeyor or your tests. We are investigating this, but it can easily happen that another xunit or .NET update will fix it before we even understand what it was.
PR was to unblock you. If you want to be able to run tests in parallel locally, but still have AppVeyor build, you can create or update AssemblyInfo.cs on the fly at the init stage of the build with say PowerShell script.