* bpo-31250, test_asyncio: fix dangling threads (#3252)
* Explicitly call shutdown(wait=True) on executors to wait until all
threads complete to prevent side effects between tests.
* Fix test_loop_self_reading_exception(): don't mock loop.close().
Previously, the original close() method was called rather than the
mock, because how set_event_loop() registered loop.close().
(cherry picked from commit 16432beadb)
* bpo-31250, test_asyncio: fix EventLoopTestsMixin.tearDown() (#3264)
Call doCleanups() to close the loop after calling
executor.shutdown(wait=True): see TestCase.set_event_loop() of
asyncio.test_utils.
Replace also gc.collect() with support.gc_collect().
(cherry picked from commit e8a533fbc7)
5013a5ebc9