[3.10] gh-99645: Fix a bug in handling class cleanups in unittest.TestCase (GH-99646) (GH-99699)

Now addClassCleanup() uses separate lists for different TestCase subclasses,
and doClassCleanups() only cleans up the particular class.

(cherry picked from commit c2102136be569e6fc8ed90181f229b46d07142f8)