drm/i915/selftests: Shorten infinite wait for sseu

Use our more regular igt_flush_test() to bind the wait-for-idle and
error out instead of waiting around forever on critical failure.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191121233021.507400-1-chris@chris-wilson.co.uk