build: Switch to headless browsers
build: Switch to headless browsers

Means that CI, libup, etc. don't need xvfb to run these tests anymore.

Change-Id: Idf52a2d5dc369f96c008349ef98c4f52220ae083

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui