asyncio: sync with Tulip

- Tulip issue 185: Add a create_task() method to event loops. The create_task()
  method can be overriden in custom event loop to implement their own task
  class. For example, greenio and Pulsar projects use their own task class. The
  create_task() method is now preferred over creating directly task using the
  Task class.
- tests: fix a warning
- fix typo in the name of a test function
- Update AbstractEventLoop: add new event loop methods; update also the unit test

Lib/asyncio/test_utils.py

@@ -48,7 +48,7 @@ def run_briefly(loop):
     def once():
         pass
     gen = once()
-    t = tasks.Task(gen, loop=loop)
+    t = loop.create_task(gen)
     # Don't log a warning if the task is not done after run_until_complete().
     # It occurs if the loop is stopped or if a task raises a BaseException.
     t._log_destroy_pending = False