rails3: fix some User#assignments_needing_grading specs

Change-Id: I0f3c3f203caf2f1b4a840d490cd4a217f70944a0
Reviewed-on: https://gerrit.instructure.com/29838
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>