drm/i915/selftests: Skip the execlists tests on !execlists machines

Ignore the tests looking at the innards of execlists and its submission
tasklets on machines that don't support execlists!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180504124202.24894-1-chris@chris-wilson.co.uk