drm/i915/selftests: Skip live_execlists if the GPU is terminally wedged

If the GPU is irrecoverably wedged, we can not execute any requests
making testing execlists (request execution) pointless. Skip!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706114510.18467-1-chris@chris-wilson.co.uk