The test suite is throttled by the headless backend repaint
timer. This commit uses the headless refresh rate option to speed up
runtime by using the immediate repaint-only-on-capture mode by
default. Tests which don't support that mode yet override the refresh
value to use the highest rate possible.
Fixes#682
Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>