[3.14] gh-129441: Fix some flakiness in test_instrumentation (gh-141881)

[miss-islington]

Most of the self.assertTrue(self.called) checks are flaky because
the worker threads may sometimes finish before the main thread calls
self.during_threads().
(cherry picked from commit 71126ab)

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

• Issue: test_free_threading: test_instrumentation() failing randomly on free-threading CI #129441