Allow to create classes in Test classes

Hi and thanks for the plugin, it's very useful.

I had some problems running single tests because of this testcase.

    def test_register_wrong_loader(self):
            class WrongLoader(object):
                pass
            with self.assertRaises(ValueError):
                resource_loader.register_loader(WrongLoader, [])

I changed a little the find_test_class method to avoid matching classes which have nothing to do with the test classes.