I encountered an issue the other day where I was unable to run unit tests in the Visual Studio 2019 Test Explorer.
The tests would just show up as 'skipped' in the Test Explorer UI, and the Test output window would say something to the effect of:
[Informational] Executing test method 'MyProject.UnitTest1.Main'
[Informational] ------ Run test started ------
[Warning] No test matches the given testcase filter `FullyQualifiedName=MyProject.UnitTest1.Main`...
[Informational] ========== Run test finished: 0 run (0:00:03.6212841) ==========
The test project used NUnit 2 for it's testing framework.
After searching the web, I found out I could add a different NuGet package that enabled running the tests from the Test Explorer.
It's called NUnitTestAdapter. After installing that package and rebuilding the project, I was able to run my tests. Though I haven't tried it, it looks like there's a separate package for running NUnit 3 tests: NUnit3TestAdapter