test_multiprocessing detects dangling per test case (#2841)

bpo-26762: test_multiprocessing now detects dangling processes and
threads per test case classes:

* setUpClass()/tearDownClass() of mixin classes now check if
  multiprocessing.process._dangling or threading._dangling was
  modified to detect "dangling" processses and threads.
* ManagerMixin.tearDownClass() now also emits a warning if it still
  has more than one active child process after 5 seconds.
* tearDownModule() now checks for dangling processes and threads
  before sleep 500 ms. And it now only sleeps if there is a least one
  dangling process or thread.