When I updated Visual Studio to the latest version, 1 of my test projects stopped running tests and outputted this message:
Test project {} does not reference any .NET N
I had a very similar issue recently with xUnit, the same outcome, however my fix was due to the fact that previously with lower versions of Microsoft.NET.Test.Sdk you didn't need XUnit.Runner.VisualStudio explicitly installed.
When I updated my Microsoft.NET.Test.Sdk to version 15.9.0 it stopped allowing tests to run until I installed the XUnit.Runner.VisualStudio Nuget.
Now, This may seem blatantly obvious, but, previously it would cope without it and still work. Now, it does not. The same will probably be true of other test platforms. It worked for me.