nondeterminstic failure of test_filesystem_with_cli_in_subprocess due to timeout

[davidsarah]

    test_filesystem_with_cli_in_subprocess ... Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/Twisted-11.0.0-py2.7-linux-x86_64.egg/twisted/internet/defer.py", line 133, in maybeDeferred
    result = f(*args, **kw)
  File "/home/zooko/playground/tahoe-lafs/pristine/src/allmydata/util/pollmixin.py", line 34, in _poll
    raise TimeoutError("PollMixin never saw %s return True" % check_f)
allmydata.util.pollmixin.TimeoutError: PollMixin never saw <bound method SystemTest._check_connections of <allmydata.test.test_system.SystemTest testMethod=tes
t_filesystem_with_cli_in_subprocess>> return True
[ERROR]

warner wrote:

there's a function-scoped variable in the test named "introducer", and a function that closes over it called _wait_until_idle(), which polls its "outstanding message" count waiting for it to drop to zero

then later, the introducer gets replaced with a new one

_wait_until_idle is still watching the old introducer, and doesn't notice the outstanding messages on the new one