test_file_descriptors_dont_leak[Nanny] is broken on MacOSX

[crusaderky]

This test deterministically leaks a file descriptor on Mac, and on Mac only:

https://github.com/dask/distributed/actions/runs/5758957886/job/15612271555

        while proc.num_fds() > before:
            await asyncio.sleep(0.01)
>           assert time() < start + 10, (before, proc.num_fds())
E           AssertionError: (26, 27)
E           assert 1691130527.5507462 < (1691130517.5493429 + 10)
E            +  where 1691130527.5507462 = time()

[crusaderky]

• XREF #4953