bpo-30357 each test in test_thread waits until all spawned threads finish (#1583)

bpo-30357 each test in test_thread waits until all spawned threads finish (#1583)

* bpo-30357 each test in test_thread waits until all spawn threads finish

* bpo-30357 each test in test_thread waits until all spawn threads finish

* bpo-30357: test_thread now uses threading_cleanup() (#1592)

test_thread: setUp() now uses support.threading_setup() and
support.threading_cleanup() to wait until threads complete to avoid
random side effects on following tests.

Co-Authored-By: Victor Stinner <victor.stinner@gmail.com>
* bpo-30357: test_thread now uses threading_cleanup() (#1592)

test_thread: setUp() now uses support.threading_setup() and
support.threading_cleanup() to wait until threads complete to avoid
random side effects on following tests.

Co-Authored-By: Victor Stinner <victor.stinner@gmail.com>