`test_close_workers` flaky

[gjoseph92]

On Windows, 3.7: https://github.com/dask/distributed/pull/4967/checks?check_run_id=2909638630#step:10:2318

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

s = <Scheduler: "tcp://127.0.0.1:60624" workers: 0 cores: 0, tasks: 0>
a = <Worker: 'tcp://127.0.0.1:60625', 0, Status.closed, stored: 0, running: 0/1, ready: 0, comm: 0, waiting: 0>
b = <Worker: 'tcp://127.0.0.1:60627', 1, Status.closed, stored: 0, running: 0/2, ready: 0, comm: 0, waiting: 0>

    @gen_cluster()
    async def test_close_workers(s, a, b):
        await s.close(close_workers=True)
>       assert a.status == Status.closed
E       AssertionError: assert <Status.closing: 'closing'> == <Status.closed: 'closed'>
E         +<Status.closing: 'closing'>
E         -<Status.closed: 'closed'>

distributed\tests\test_scheduler.py:1799: AssertionError

(I know I have seem the same failure a few times before.)

[mrocklin]

cc @fjetter who was working on this recently

[fjetter]

Yes, I've stumbled over this recently. I believe this is a race condition between nanny shutdown and server shutdown. I think it was in #4734