[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.

[test] Use `with open` syntax in cwconfig "prefix" tests