[3.12] gh-131677: Fix flaky test_lru_cache_threaded3 (gh-131679) (gh-131693)

The call to `with self.subTest(...)` was not thread-safe.
(cherry picked from commit a1232459860235f4fb7896cc95966d87a51cbe32)

Co-authored-by: Sam Gross <colesbury@gmail.com>