drm/i915/selftests: Teach execlists to take intel_gt as its argument

The execlists selftests are hardware centric and so want to use the gt
as its target.

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/20191016120249.22714-1-chris@chris-wilson.co.uk