Accelerate tests

From this record on travis-ci these are our slowest tests:

```
========================== slowest 20 test durations ===========================
41.75s call distributed/tests/test_scheduler.py::test_file_descriptors
35.96s call distributed/tests/test_stress.py::test_close_connections
20.66s call distributed/tests/test_stress.py::test_cancel_stress_sync
15.86s call distributed/tests/test_client.py::test_open_close_many_workers[Worker-100-5]
13.79s call distributed/tests/test_client.py::test_open_close_many_workers[Nanny-10-20]
13.57s call distributed/tests/test_stress.py::test_cancel_stress
12.33s call distributed/tests/test_stress.py::test_stress_creation_and_deletion
10.35s call distributed/tests/test_variable.py::test_race
10.19s call distributed/tests/test_client.py::test_missing_worker
9.80s call distributed/tests/test_client.py::test_threadsafe_get
9.41s call distributed/tests/test_client.py::test_threadsafe_compute
7.74s call distributed/deploy/tests/test_local.py::test_defaults
7.40s call distributed/tests/test_asyncio.py::test_asyncio_restart
6.92s call distributed/tests/test_stress.py::test_stress_gc[func1-1000]
6.87s call distributed/tests/test_client.py::test_secede_balances
5.88s call distributed/tests/test_worker.py::test_worker_death_timeout
5.08s call distributed/tests/test_queues.py::test_race
4.98s call distributed/tests/test_nanny.py::test_nanny_terminate
4.63s call distributed/tests/test_scheduler.py::test_non_existent_worker
4.41s call distributed/tests/test_worker_failure.py::test_gather_then_submit_after_failed_workers
```

It may be worth looking at a few of the slower ones to see if they can be accelerated.