Stop Minion processing once signaled.

This is, apparently, the problem with the aborted Jenkins builds.
The `runtests.py` script runs the minion in a `multiprocess` and tries
to `join()` it after `terminate()` is called.
This problem does not show up on every execution and, so far, I wasn't
able to narrow down the culprit code because of the weak reproducibility, but this, however, seems to stop the problem from reappearing.