Test suite leaks many objects

[pitrou]

At the end of the test suite (I'm running distributed/tests):

>>> l = [x for x in gc.get_objects() if isinstance(x, (distributed.Scheduler, distributed.Client, distributed.Worker, distributed.Nanny))]
>>> len(l)
479
>>> pprint.pprint(collections.Counter(type(x) for x in l))
Counter({<class 'distributed.scheduler.Scheduler'>: 447,
         <class 'distributed.worker.Worker'>: 19,
         <class 'distributed.client.Client'>: 9,
         <class 'distributed.nanny.Nanny'>: 4})

So the test suite is really leaking a ton of Scheduler instances...

[GenevieveBuckley]

Is this a duplicate of #1597, or are there differences between the two?