spec: slighty de-insanify assignment_model factory

refs CNVS-16239

Change-Id: If6f9cba61a01a1cc60f0b63b581a813a076b9b02
Reviewed-on: https://gerrit.instructure.com/42853
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>