drm/i915/gt: add selftest to exercise WABB

Test re-uses logic form indirect ctx BB selftest.

Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231026-wabb-v6-3-4aa7d55d0a8a@intel.com