[3.12] gh-116682: stdout may be empty in test_cancel_futures_wait_false (GH-116683) (#116692)

If the `shutdown()` call happens before the worker thread starts executing
the task, then nothing will be printed to stdout.
(cherry picked from commit 7d1abe9502641a3602e9773aebc29ee56d8f40ae)

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