remove ValueError from ClockEventLoop.advance_time in favor of returnning immediately

Author: derekbrokeit

pytest_asyncio/plugin.py

@@ -212,7 +212,8 @@ async def advance_time(self, seconds):
         Advance time by a given offset in seconds.
         '''
         if seconds < 0:
-            raise ValueError('cannot go backwards in time')
+            # cannot go backwards in time, so return immediately
+            return
 
         # advance the clock by the given offset
         self._offset += seconds