`test_map` flaky

On macOS, 3.9: https://github.com/dask/distributed/runs/2947008056#step:10:2261

```python
=================================== FAILURES ===================================
___________________________________ test_map ___________________________________

client =

def test_map(client):
with client.get_executor() as e:
N = 10
it = e.map(inc, range(N))
expected = set(range(1, N + 1))
for x in it:
expected.remove(x)
assert not expected

with client.get_executor(pure=False) as e:
N = 10
it = e.map(slowinc, range(N), [0.1] * N, timeout=0.4)
results = []
with pytest.raises(TimeoutError):
for x in it:
results.append(x)
> assert 2 <= len(results) < 7
E assert 2 <= 1
E + where 1 = len([1])

distributed/tests/test_client_executor.py:155: AssertionError
```

I assume the issue here is that CI was somehow slow enough that not enough 0.1sec tasks were able to run in the 0.4sec timeout.