Layered test checker doesn't diagnose three-layer cakes

[nedbat]

This code runs two tests in nose and pytest:

# Three-layer cake.

class BaseTest(unittest.TestCase):
    __test__ = False
    def test_it(self):
        self.fail("Hello, world!")

class Middle(BaseTest):
    __test__ = True

class MoreTests(Middle):
    __test__ = True

but the layered test checker doesn't detect it.