[test/fix] prevent FirefoxHelper from silently failing to start
This was breaking some tests without any obvious error message and you should
fail as early as possible to make debugging easier.
Closes #17260390
extend-diff-ignore = "^(__pkginfo__\.py$|doc/_themes/cubicweb/static/(logo-cubicweb-small\.svg|cubicweb\.ico|logo-cubicweb\.svg)$|cubicweb\.spec$|setup\.cfg$|cubicweb/misc/cwfs)"