bpo-43913: Fix bugs in cleaning up classes and modules in unittest. (GH-28006)

* Functions registered with addModuleCleanup() were not called unless
  the user defines tearDownModule() in their test module.
* Functions registered with addClassCleanup() were not called if
  tearDownClass is set to None.
* Buffering in TestResult did not work with functions registered
  with addClassCleanup() and addModuleCleanup().
* Errors in functions registered with addClassCleanup() and
  addModuleCleanup() were not handled correctly in buffered and
  debug modes.
* Errors in setUpModule() and functions registered with
  addModuleCleanup() were reported in wrong order.
* And several lesser bugs.