gh-97669: Fix test_tools reference leak (#98216)

test_tools.test_sundry() now uses an unittest mock to prevent the
logging module to register a real "atfork" function which kept the
logging module dictionary alive. So the logging module can be
properly unloaded. Previously, the logging module was loaded before
test_sundry(), but it's no longer the case since recent test_tools
sub-tests removals.

Lib/test/support/import_helper.py

@@ -246,3 +246,11 @@ def modules_cleanup(oldmodules):
     # do currently). Implicitly imported *real* modules should be left alone
     # (see issue 10556).
     sys.modules.update(oldmodules)
+
+
+def mock_register_at_fork(func):
+    # bpo-30599: Mock os.register_at_fork() when importing the random module,
+    # since this function doesn't allow to unregister callbacks and would leak
+    # memory.
+    from unittest import mock
+    return mock.patch('os.register_at_fork', create=True)(func)