Depends on D17054
The previous version only relied on a manifest, but it's not really flexible.
The XPI approach allows to define background scripts, which can be useful for
more tests.
Note: the old about:debugging supports both the manifest and the XPI approaches
but we should try to keep only one here to make it less complicated to write new
tests.
Differential Revision: https://phabricator.services.mozilla.com/D17055
--HG--
extra : moz-landing-system : lando
