DA - refactor assignment_spec.rb

fixes CNVS-15973

test plan:
 - tests pass

Change-Id: I83a9ff880e170c078528a2efc564b897c32b4d07
Reviewed-on: https://gerrit.instructure.com/42316
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>