Speed up tests runtime

Version 1:

This MR exposes a new repaint API implemented by the headless backend to customise the framerate. It is used here to speed up tests runtime but could later be used to expose new repaint strategies.

molinari/weston:speed-up-tests-runtime-v1

Version 2:

This MR exposes a new frontend CLI option to customise the refresh rate of the headless backend and possibly switch to a repaint on capture only mode. It is used here to speed up tests runtime.

Fixes #682 (closed)