[2.7] bpo-31234: Join threads explicitly in tests (#7406)

[2.7] bpo-31234: Join threads explicitly in tests (#7406)

* Add support.wait_threads_exit(): context manager looping at exit
  until the number of threads decreases to its original number.
* Add some missing thread.join()
* test_asyncore.test_send(): call explicitly t.join() because the cleanup
  function is only called outside the test method, whereas the method
  has a @test_support.reap_threads decorator
* test_hashlib: replace threading.Event with thread.join()
* test_thread:

  * Use wait_threads_exit() context manager
  * Replace test_support with support
  * test_forkinthread(): check child process exit status in the
    main thread to better handle error.