drm/i915/selftests: Terminate hangcheck sanitycheck forcibly

If all else fails and we are stuck eternally waiting for the undying
request, abandon all hope.

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/20181203113701.12106-6-chris@chris-wilson.co.uk