(Merge 3.4) asyncio: Set __qualname__ attribute of CoroWrapper in @coroutine

decorator on Python 3.5.

- Drop __slots__ optimization of CoroWrapper to be able to set the __qualname__
  attribute.
- Add tests on __name__, __qualname__ and __module__ of a coroutine function
  and coroutine object.
- Fix test_tasks when run in debug mode (PYTHONASYNCIODEBUG env var set) on
  Python 3.3 or 3.4