def METHOD_NAME(self):
    with self.subTest("Register not immutable"):
        MockPayable.immutable_after_payment = False
        payables.register(MockModel, MockPayable)
    with self.subTest("Register immutable"):
        MockPayable.immutable_after_payment = True
        MockPayable.immutable_model_fields_after_payment = [
            "test_field",
        ]
        payables.register(MockModel, MockPayable)