gh-94808: add tests covering PyFunction_{Get,Set}Closure

[sobolevn]

It was like this:

Now all branches are covered.

• Issue: Metabug: Improving C-level coverage #94808

[kumaraditya303]

Suggested change

	Py_INCREF(closure);
	return closure;
	return Py_NewRef(closure);

[kumaraditya303]

This has merge conflicts now.

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

No more merge conflicts

[encukou]

After a year, I took the liberty of updating to the current main branch, and fixed two nitpicks.

[gvanrossum]

Sorry about that. Go ahead and merge!

[sobolevn]

@encukou thanks a lot for your help! I forgot about this PR a long time ago :)

[encukou]

No problem, happy to help :)

Usually we backport test changes to reduce future conflicts, but since this already causes conflicts in 3.12, I'll skip the backport here.