[devtools/qunit] Turn FirefoxHelper into a context manager So that stop() method always gets called. Also ensure temporary directory created on __init__ gets deleted as well upon exit.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/python2*/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl