Flaky tests: plain `TimeoutError` during client connection to scheduler

Manifests as a CancelledError from `Comm.read`:
```
> frames_nbytes = await stream.read_bytes(fmt_size)
E asyncio.exceptions.CancelledError
```
Which eventually comes out as a `TimeoutError` from `asyncio.wait_for`:
```
except exceptions.CancelledError as exc:
> raise exceptions.TimeoutError() fromexc
E asyncio.exceptions.TimeoutError
```
Then, tearing down the test times out after 10s waiting for all global clients to close:
```
whileset(_global_clients):
sleep(0.1)
> assert time() < start + 10
E assert 1657573967.6341991 < (1657573957.571699 + 10)
E + where 1657573967.6341991 = time()
```

I would imagine the underlying cause is related to (or even the same as) https://github.com/dask/distributed/issues/6731.