Flaky tests: plain TimeoutError during client connection to scheduler

[gjoseph92]

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) #6731.

[gjoseph92]

• Flaky test_no_dashboard #6756
• Flaky test_scheduler_port_zero #6758