Flaky `test_quiet_client_close`

This hasn't actually failed yet on the test report, but I know it could like it did here: https://github.com/dask/distributed/pull/6504#issuecomment-1150423010

This is caused by https://github.com/dask/distributed/issues/6390 in fallout from https://github.com/dask/distributed/pull/6361.

```python
def test_quiet_client_close(loop):
with captured_logger(logging.getLogger("distributed")) as logger:
with Client(
loop=loop,
processes=False,
dashboard_address=":0",
threads_per_worker=4,
) as c:
futures = c.map(slowinc, range(1000), delay=0.01)
sleep(0.200) # stop part-way
sleep(0.1) # let things settle

out = logger.getvalue()
lines = out.strip().split("\n")
assert len(lines) <= 2
for line in lines:
> assert (
not line
or "Reconnecting" in line
or "garbage" in line
or set(line) == {"-"}
), line
E AssertionError: Received heartbeat from unregistered worker 'inproc://10.213.1.205/15971/24'.
```

I think we can just swap `reconnecting` with `unregistered worker` in the acceptable output. It's not great that that happens, but the only fix for it is https://github.com/dask/distributed/issues/6390. So in the interim, we can just accept that it may happen.