drm/i915/selftests: Add the mock engine to the gt->engine[]

Remember to include the newly created mock engine in the list of
available engines inside the gt.

Fixes: a50134b1

 ("drm/i915: Make for_each_engine_masked work on intel_gt")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191018130703.31125-1-chris@chris-wilson.co.uk