Add support for executing tests in concrete sub-classes when abstract class is targeted

[jcansdale]

E.g. 2 tests should pass when AbstractTests or AbstractTests.Test is targeted.

    public abstract class AbstractTests
    {
        [Test]
        public void Test() { }
    }

    public class PassConcreteTests : AbstractTests
    {
    }

    public class FailConcreteTests : AbstractTests
    {
    }