test_restart_during_computation flaky

[gjoseph92]

On Windows, 3.8: https://github.com/dask/distributed/pull/4925/checks?check_run_id=2888102603#step:10:2445

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

check = True

    @contextmanager
    def check_process_leak(check=True):
        for proc in mp_context.active_children():
            proc.terminate()
    
        yield
    
        if check:
            for i in range(200):
                if not set(mp_context.active_children()):
                    break
                else:
                    sleep(0.2)
            else:
>               assert not mp_context.active_children()
E               AssertionError: assert not [<Process name='Dask Worker process (from Nanny)' pid=5616 parent=2984 started daemon>]
E                +  where [<Process name='Dask Worker process (from Nanny)' pid=5616 parent=2984 started daemon>] = <function active_children at 0x000001CC8C16BE50>()
E                +    where <function active_children at 0x000001CC8C16BE50> = mp_context.active_children

distributed\utils_test.py:1472: AssertionError

[hendrikmakait]

test_restart_during_computation has not appeared as a flaking test on the test report for the last 50 workflows, so this seems to be fixed.