'Assert that get_date returns valid date.'

def test_get_date(self):

    ccx = self.make_ccx()
    for section in self.course.get_children():
        self.assertEqual(get_date(ccx, section, 'start'), self.mooc_start)
        self.assertEqual(get_date(ccx, section, 'due'), None)
        for subsection in section.get_children():
            self.assertEqual(get_date(ccx, subsection, 'start'), self.mooc_start)
            self.assertEqual(get_date(ccx, subsection, 'due'), self.mooc_due)
            for unit in subsection.get_children():
                self.assertEqual(get_date(ccx, unit, 'start', parent_node=subsection), self.mooc_start)
                self.assertEqual(get_date(ccx, unit, 'due', parent_node=subsection), self.mooc_due)
