[IMP] tests: partially backport classCleanups from CPython 3.8

This commit partially backports bpo-24412, which allows the definition of
class cleanups (addClassCleanup) and module cleanups (omitted),
similar to instance cleanups (addCleanup).

This is useful for tests that override unittest's setUpClass and
could crash during its execution: If this happens, it is possible that a
bunch of crap is left in the database or even worse, the cursor becomes
completely fucked; Thanks to the addClassCleanup, we can undo the damage
done by the setUpClass.

Another benefit is that it is called unconditionally after tearDownClass
is called, so it can also be called as a replacement and/or safer
tearDownClass.