'Test the unit properties are returned as expected.'

def test_properties(self):

    self.assertEqual(LENGTH_KILOMETERS, METRIC_SYSTEM.length_unit)
    self.assertEqual(TEMP_CELSIUS, METRIC_SYSTEM.temperature_unit)
    self.assertEqual(MASS_GRAMS, METRIC_SYSTEM.mass_unit)
    self.assertEqual(VOLUME_LITERS, METRIC_SYSTEM.volume_unit)
