Add `addCleanup` to `unittest.subTest`

Feature or enhancement

Proposal:

The unittest.subTest context (i.e. the object you get using with's as clause, currently None) should get methods to manage cleanups:

• addCleanup
• enterContext
• doCleanups

And similar ones for IsolatedAsyncIOTestCase.

They should do the same thing as the same methods on TestCase, but with subtest scope.
Example usage:

for param in 'a', 'b', 'c':
    with self.subTest() as sub:
        tempfile = make_tempfile()
        sub.addCleanup(os.unlink, tempfile)
        do_actual_test(tempfile, param)

See Discuss thread for motivation/discussion.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/unittest-add-addcleanup-to-subtest/91827

Linked PRs

• gh-134318

Start with the existing unittest.TestCase cleanup methods and the unittest.subTest context, then compare the corresponding IsolatedAsyncIOTestCase behavior. Read the Discourse thread and linked PR gh-134318 to understand the proposed scope. Done means the requested cleanup methods work with subtest scope for both synchronous and isolated asynchronous cases.