Flaky test_avoid_paused_workers

flaky `TimeoutError` in `test_avoid_paused_workers`

main ubu py3.8 https://github.com/dask/distributed/runs/4552302807?check_suite_focus=true

feature branch win py3.7 https://github.com/dask/distributed/runs/4537729477?check_suite_focus=true

```
E asyncio.exceptions.TimeoutError: Test timeout after 30s.
E Stack for wait_for=()]>> (most recent call last):
E File "/home/runner/work/distributed/distributed/distributed/tests/test_scheduler.py", line 3235, in test_avoid_paused_workers
E await asyncio.sleep(0.01)
```