Add shutdown crash tests

[saschanaz]

AFAIK WPT does not shuts the process down after a crashtest, and I've seen many cases in Mozilla where we opted not to write WPT solely because of that. Could be great if there is either:

• A type of crashtest that always shuts down the process after finished
• Or a mode that enables the behavior for all crashtests

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1766711 and web-platform-tests/wpt#34256. (Duplicating here for me to write an RFC...)