I am using NUnit, mostly because I'm most familiar with this test framework. There's not a lot of tests for the code itself, it is mostly used for testing things out before implementation.

You can use the regular $ dotnet test command to run the tests without any additional tools.

If you want to contribute new test code, I have a couple of preferences:

Do use Assert.That(expr, Is/Does/etc) format instead of deprecated Assert.AreEqual() and similar.
Try to write the code in the way that does not require the use of InternalsVisibleTo attribute.
Tests that require any external data that must be manually supplied, should be disabled by default.