I don't think this is worth spending time adding tests for. I get what you're saying that this is at least semi-intentional behaviour and desirable behaviour so it should have tests ensuring that it continues to work. But it's not documented behaviour and the test is basically testing that the implementation is this specific implementation.
I don't think the test is really a bad idea but neither is it really very useful and I think it's not worth having people spend time reviewing and discussing. I'm leaning to saying this patch is rejected.
