Consider making NopFilter public

[thomhurst]

Currently NopFilter is internal. However it's the default filter being passed in if doing a basic dotnet run on your test suite.

If we switch on the Type of an ITestExecutionFilter, we're unable to see the NopFilter.

I think it should be public so we can see it when checking the type of filter we've been passed.

[MarcoRossignoli]

like explained in the guide the filter is in experimental internal mode https://github.com/microsoft/testfx/blob/main/docs/testingplatform/irequest.md#discovertestexecutionrequest but I'd like to find a way to let you use it and feedback us and help to shape it

linked to #2462

cc: @Evangelink