I reverted the change to test_logging, reran, and it runs fine, at least twice. Before the revert, I checked that the only two changes were the intentional ones.

I tried adding idle_test/test_zdummy.py with a tearDownModule that deletes sys.modules['warnings']. This 'works' in that the original failure passes, as with the change to test_logging, and
'dummy.py:42: UserWarning: Explicit' is printed. But with test_logging unchanged from when it ran and passed, I see the same 0 != 1 error as in the last message plus

Traceback (most recent call last):
  File "F:\Python\dev\cpython\lib\test\test_logging.py", line 1757, in test_warnings
    self.assertGreater(s.find("UserWarning: I'm warning you...\n"), 0)
AssertionError: -1 not greater than 0