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

Comienza con los métodos de limpieza existentes de unittest.TestCase y el contexto unittest.subTest; después, compara el comportamiento correspondiente de IsolatedAsyncIOTestCase. Lee el hilo de Discourse y el PR vinculado gh-134318 para entender el alcance propuesto. Se considera terminado cuando los métodos de limpieza solicitados funcionan con el ámbito de subtest tanto para casos síncronos como para casos asíncronos aislados.