[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.
[test] Replace logilab-common's with_tempdir by tempfile.TemporaryDirectory
logilab-common's with_tempdir decorator, which is considered harmful (see
https://www.logilab.org/ticket/8267966).
Rely on backports.tempfile to provide TemporaryDirectory on Python 2.
Re-export it in testlib module for convenience.