drm/i915/selftests: Force a failed engine reset

Inject a fault into the engine reset and check that the outstanding
requests are completed despite the failed reset.

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/20210113205049.22565-1-chris@chris-wilson.co.uk