bpo-45678: Add more ``singledispatchmethod`` tests (GH-29412)

In order to fix a bug in the 3.9 branch in #29394, more tests were added to
``test_functools.py`` to ensure that ``singledispatchmethod`` still correctly
wrapped a target method, even if the target method had already been wrapped by
 multiple other decorators. This PR brings the new tests into the 3.11 and 3.10
branches as well.