[3.14] gh-129441: Fix some flakiness in test_instrumentation (gh-141881) (gh-141913)

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 71126ab19c76d8227fc3b9c7c59b957ca82af4dd)

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