I tried adding a very simple unit test project in my solution. As per document here https://docs.microsoft.com/en-us/dotnet/core/testing/unit-testing-with-nunit, added NUnit, NUnit3TestAdapter, Microsoft.NET.Test.Sdk (this was added for me).
Note that if you r-c on your solution > Add new project > .NetCore > Test > XUnit, appropriate XUnit packages are added. So, I tried both for NUnit and XUnit.
In either case, test runners detect test and I can run them just fine; however, debugging test hangs VS4Mac