'Tests `get_parent_unit` method for the successful result.'

def test_get_parent_unit(self):

    parent = utils.get_parent_unit(self.html_module_1)
    self.assertEqual(parent.location, self.vertical.location)
    parent = utils.get_parent_unit(self.child_html_module)
    self.assertEqual(parent.location, self.vertical_with_container.location)
    self.assertIsNone(utils.get_parent_unit(None))
    self.assertIsNone(utils.get_parent_unit(self.vertical))
    self.assertIsNone(utils.get_parent_unit(self.course))
    self.assertIsNone(utils.get_parent_unit(self.chapter))
    self.assertIsNone(utils.get_parent_unit(self.sequential))
