simplify EventLoopClockAdvancer and clarify __call__ method

pytest_asyncio/plugin.py

@@ -168,6 +168,7 @@ def pytest_runtest_setup(item):
 
 class EventLoopClockAdvancer:
     __slots__ = ("offset", "_base_time",)
+
     def __init__(self, loop):
         self.offset = 0.0
         self._base_time = loop.time
@@ -176,14 +177,17 @@ def __init__(self, loop):
     def time(self):
         return self._base_time() + self.offset
 
-    def __call__(self, seconds):
+    async def __call__(self, seconds):
+        # sleep so that the loop does everything currently waiting
+        await asyncio.sleep(0)
+
         if seconds > 0:
             # advance the clock by the given offset
             self.offset += seconds
 
-        # Once the clock is adjusted, new tasks may have just been
-        # scheduled for running in the next pass through the event loop
-        return self.create_task(asyncio.sleep(0))
+            # Once the clock is adjusted, new tasks may have just been
+            # scheduled for running in the next pass through the event loop
+            await asyncio.sleep(0)
 
 
 @pytest.yield_fixture