xUnit tests fails when executed by Azure DevOps

The problem was that the default file pattern to look for test assemblies (*test*.dll) also did include xUnit's core assemblies (named xunit.runner.visualstudio.dotnetcore.testadapter.dll), which confused the test system.

All I had to do to fix the issue is use a more specific pattern (like *tests.dll), or exclude all test adapter assemblies (!**/*testadapter.dll).

Update : The official xUnit documentation has been updated to help avoid this problem (https://xunit.net/docs/getting-test-results-in-azure-devops)