Travis tests intermittently fail with an extra process error

[s1113950]

While working on #658 I noticed this would happen sometimes:

======================================================================
ERROR: tearDownClass (doas_test.DoasTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/dw/mitogen/tests/testlib.py", line 529, in tearDownClass
    cls.dockerized_ssh.check_processes()
  File "/home/travis/build/dw/mitogen/tests/testlib.py", line 478, in check_processes
    counts
AssertionError: Docker container 'mitogen-test-e71a28532b8fc46f' contained extra running processes after test completed: {u'doas <defunct>': 1, u'ps': 1, u'sshd': 1}

Example:
90d51fc this commit passed travis
The commit right after to remove debug logging again failed travis: e138286
even though previously without -vvv travis passed here: e85720f

It seems unrelated to changes made in the PR, but will need to investigate further to be sure

[moreati]

Duplicates #694