Test take_action() in unit tests

[Tang Chen]

Some of the unit tests test run(), but not take_action().

For example, in openstackclient/tests/volume/v1/test_volume.py, there is:

    def test_volume_set_size_smaller(self):
        ......

        result = self.cmd.run(parsed_args)

        self.assertEqual(0, result)
        ......

run() is defined in class Command in cliff. We don't need to test it in OSC unit tests. On the contrary, we should test take_action(), which is overwritten in each command classes in OSC.