add asyncio_clock mark so that coroutines can be run in a ClockEventLoop explicitly

Author: derekbrokeit

pytest_asyncio/plugin.py

@@ -196,6 +196,7 @@ async def advance_time(self, seconds):
 # to marked test functions
 _markers_2_fixtures = {
     'asyncio': 'event_loop',
+    'asyncio_clock': 'clock_event_loop',
 }