backport stable

[devtools] make a copy of the xvfb-run script so we can kill it properly Since changeset a3d3bdd46463 we don't run firefox directly but through a wrapper script. This meant that on stopping the test, we would kill that wrapper script but leave the Xvfb server and firefox processes running. So make a local copy of xvfb-run, and make it kill Xvfb when it goes down, not just when its client exits.

[ajax] use AJAX_BASE_URL instead of JSON_BASE_URL JSON_BASE_URL / 'json' should be replaced by AJAX_BASE_URL / 'ajax'