bitmask-dev tests hang when failing in docker runner
see https://0xacab.org/varac/bitmask-dev/builds/2560, I had to cancel after a long time. (question is also, why do the tests run locally for me and not in docker CI, but this is a different thing).
This looks similar, and holger even answered: https://stackoverflow.com/questions/19219596/py-test-hangs-after-showing-test-results
(from redmine: created on 2016-10-07, closed on 2016-11-08, relates #8528 (closed))