racey test canonical/lazr/doc/pidfile.txt: "pid file was not removed"

when trying to land https://code.launchpad.net/~mbp/launchpad/881237-dkim-error/+merge/80413

I hit

Failing tests
-------------
 pidfile_txt

======================================================================
FAILURE: pidfile_txt (subunit.RemotedTestCase)
----------------------------------------------------------------------
_StringException: Text attachment: traceback
------------
Traceback (most recent call last):
 File "/usr/lib/python2.6/unittest.py", line 279, in run
   testMethod()
 File "/usr/lib/python2.6/doctest.py", line 2152, in runTest
   raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for pidfile.txt
 File "/var/launchpad/test/lib/canonical/lazr/tests/../doc/pidfile.txt", line 0

----------------------------------------------------------------------
File "/var/launchpad/test/lib/canonical/lazr/tests/../doc/pidfile.txt", line 113, in pidfile.txt
Failed example:
   stop(pid, signal.SIGTERM)
Differences (ndiff with -expected +actual):
   - Stopped successfully
   + Error: pid file was not removed

------------

Muharem hit the same problem back in 2009, according to my mail archive, so I guess it's a timing dependent bug.