[3.12] Fix test_importlib.test_side_effect_import() (GH-104840) by miss-islington · Pull Request #104842 · python/cpython

Wait until the thread spawn by the import completes to avoid dangling
threads. With this fix, the following command no longer fails:

./python -m test --fail-env-changed test_importlib -m test_side_effect_import -F -j20
(cherry picked from commit 4269509)

Co-authored-by: Victor Stinner <vstinner@python.org>