(tests) Cancellation needs another yield

test.py

@@ -26,9 +26,6 @@ def task(mode):
 async def mutate_tasks_in_sequence(task_states, *funcs):
     history = []
 
-    async def null(_):
-        pass
-
     for func in funcs + (null, ):
         await asyncio.sleep(0)
         await asyncio.sleep(0)
@@ -50,6 +47,10 @@ async def func(tasks):
     return func
 
 
+async def null(_):
+    pass
+
+
 def async_test(func):
     def wrapper(*args, **kwargs):
         future = func(*args, **kwargs)
@@ -121,7 +122,7 @@ async def test_mutex_cancelled_before_it_starts_allows_later_mutex(self):
         started_history = await mutate_tasks_in_sequence(create_lock_tasks(
             lock,
             Mutex, Mutex, Mutex),
-            cancel(1), complete(0), complete(2),
+            cancel(1), complete(0), null, complete(2),
         )
 
         self.assertEqual(started_history[0], [True, False, False])