[3.6] bpo-31250: test_asyncio: fix dangling threads (#3517)

[3.6] bpo-31250: test_asyncio: fix dangling threads (#3517)

* 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 16432beadb8eba079c9786cc0c0eaacfd9fd2f7b)

* 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 e8a533fbc734af6eeb389202ba6c6e9c2548027f)