flaky test `distributed/tests/test_asyncprocess.py::test_exit_callback`

seen in https://github.com/dask/distributed/runs/7217310819?check_suite_focus=true#step:11:1824

```
______________________________ test_exit_callback ______________________________
@gen_test()
asyncdeftest_exit_callback():
to_child = get_mp_context().Queue()
from_child = get_mp_context().Queue()
evt = Event()
# FIXME: this breaks if changed to async def...
@gen.coroutine
defon_stop(_proc):
assert _proc is proc
yield gen.moment
evt.set()
# Normal process exit
proc = AsyncProcess(target=feed, args=(to_child, from_child))
evt.clear()
proc.set_exit_callback(on_stop)
proc.daemon = True
await proc.start()
await asyncio.sleep(0.05)
assert proc.is_alive()
assertnot evt.is_set()
to_child.put(None)
> await evt.wait(timedelta(seconds=5))
E tornado.util.TimeoutError: Timeout
distributed/tests/test_asyncprocess.py:262: TimeoutError
- generated xml file: /Users/runner/work/distributed/distributed/reports/pytest.xml -
```