Add clock event loop as default

[derekbrokeit]

Extend #96 clock_event_loop fixture to become default event_loop

• uses a custom policy meta class that uses the existing policy to
  modify the new loop it creates.
• The new loop it creates is modified to provide advance_time
  coroutine for testing
• Extends Add ClockEventLoop class with fixture and test (close #95) #96 to make ClockEventLoop the default policy for testing

I am adding this as a separate merge request because I am uncertain if making this behavior default is desirable, so it is here for your consideration. As I mentioned in #96, if this behavior is to be as a separate event-loop, major refactoring to the pytest_fixture_setup and similar methods will be needed since they all assume event_loop is the name of the fixture for event-loops. There is currently no trivial way to allow custom event-loops in pytest-asyncio as far as I can tell. This is the reason #96 fails many of the tests.

[derekbrokeit]

Looking at this implementation, the same utilities here can actually be refactored out to a clean fixture as suggested in #83 if desired. If desired, the advance_time method used here could be broken out into a fixture to remove the need to change the underlying loop policy and class, which might be a cleaner solution.

[derekbrokeit]

Closing this because #113 is a better solution in my opinion