Test to exercise Callbacks#reset_callbacks for descendants

Here is the patch

Yeah, you're right perhaps it's better to encapsulate this test into its own class.

Here I attach a patch implementing that suggestion.

What I had originally meant was instead of modifying CleanPerson class, writing a completely new one for this test only. To be honest though, I don't think either what I suggested or what you've done is really needed. I think it's good to go.

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