flaky test_last_in_first_out[queue on worker]

```
___________________ test_last_in_first_out[queue on worker] ____________________

c =
s =
a =
pause = False

@gen_blockable_cluster
async def test_last_in_first_out(c, s, a, pause):
async with block_worker(c, s, a, pause, 15, 5):
xs = [c.submit(slowinc, i, delay=0.05, key=f"x{i}") for i in range(5)]
ys = [c.submit(slowinc, xs[i], delay=0.05, key=f"y{i}") for i in range(5)]
zs = [c.submit(slowinc, ys[i], delay=0.05, key=f"z{i}") for i in range(5)]

while not any(s.tasks[z.key].state == "memory" for z in zs):
await asyncio.sleep(0.01)
> assert not all(s.tasks[x.key].state == "memory" for x in xs)
E assert not True
E + where True = all(. at 0x13887e500>)

distributed/tests/test_priorities.py:233: AssertionError
```

https://github.com/dask/distributed/runs/6176976855?check_suite_focus=true#step:11:1642