D2LTESTS004 failure on an abstract test fixture base

[mthjones]

When adding an abstract test fixture base class that contains [Test] methods to a test framework project, D2LTESTS004 analyzer errors are reported on the test, despite the class itself not being marked with [TestFixture].

e.g. Putting this in a project marked as a test framework will fail the analyzer.

public abstract class FooBase {
    [Test]
    public void FooTest() {}
}

r? @omsmith