[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/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc