'Test errors are raised when invalid units are passed in.'

def test_invalid_units(self):

    with self.assertRaises(ValueError):
        UnitSystem(SYSTEM_NAME, INVALID_UNIT, LENGTH_METERS, VOLUME_LITERS, MASS_GRAMS)
    with self.assertRaises(ValueError):
        UnitSystem(SYSTEM_NAME, TEMP_CELSIUS, INVALID_UNIT, VOLUME_LITERS, MASS_GRAMS)
    with self.assertRaises(ValueError):
        UnitSystem(SYSTEM_NAME, TEMP_CELSIUS, LENGTH_METERS, INVALID_UNIT, MASS_GRAMS)
    with self.assertRaises(ValueError):
        UnitSystem(SYSTEM_NAME, TEMP_CELSIUS, LENGTH_METERS, VOLUME_LITERS, INVALID_UNIT)
