The link in accepted answer is not working, so I am adding my solution of the problem.

Using xUnit 2.8.1 and visual studio runner.

When I tried to disable parallelization in xunit.runner.json, it did not work. Neither with just

"parallelizeTestCollections": false

nor when adding

"maxParallelThreads": 1

my solution was to use the assembly attribute

[assembly: CollectionBehavior(MaxParallelThreads = 1, DisableTestParallelization = true)]