gh-145678: Guard against re-entrant __eq__ never being called

Add assertIsNotNone check so the test fails explicitly if the
re-entrant code path is never triggered, rather than passing vacuously.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sampsonc

Lib/test/test_itertools.py

@@ -780,6 +780,7 @@ def __hash__(self):
         g = itertools.groupby([1, 1, 2], lambda _: next(keys_iter))
         k, grp = next(g)
         list(grp)  # must not crash with address sanitizer
+        self.assertIsNotNone(ref_before, "re-entrant __eq__ was never called")
         # On an unpatched build, next(g) releases gbo->currkey's reference to
         # 'other', so the count drops by 1 -- the object will be freed when the
         # __eq__ frame exits, leaving do_richcompare() with a dangling pointer.