drm/i915/selftests: Refactor common flush_test()

Pull igt_flush_test() out into its own library before copying and
pasting the code for a third time.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180505091014.26126-1-chris@chris-wilson.co.uk