Flaky test_worker_waits_for_scheduler

This test hang twice, both times on Windows, over a 120 runs stress test:
https://github.com/crusaderky/distributed/runs/5308271287?check_suite_focus=true
There's nothing interesting in the stderr and sadly since it's not a ``@gen_cluster`` test there is no dump.

I can't recall why, in #5060, I felt the need to replace
```
< @pytest.mark.asyncio
> @gen_test(timeout=60)
```
with an explicit timeout, but I guess that I observed it hang and I thought it would take longer than 30s. It doesn't - it's supposed to take 3~3.5s every time.