tests: fix race condition, take two
tests: fix race condition, take two

As noted in da884497e5, that attempted a somewhat poor fix.  This of
course turned out not to be good enough to succeed on travis-ci.

This should now really fix #260.

Signed-off-by: Yung-Chin Oei <e4c1ab9da74060df152545062ee274845648ed6c@yungchin.nl>

wikimedia/operations-debs-python-pykafka,wikimedia/operations-debs-python-pykafka,benauthor/pykafka,thedrow/samsa,thedrow/samsa,wikimedia/operations-debs-python-pykafka,benauthor/pykafka,thedrow/samsa,benauthor/pykafka