support scopes for find_ids_in_batches

we already use it that way, and it makes the spec less fragile

Change-Id: Ia38ee9d735a7d2652565edbb46f1dbb11bfdd879
Reviewed-on: https://gerrit.instructure.com/29184
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>