spec: fix assignments_student_spec for draft state

refs #CNVS-15563

Change-Id: I3c62392c421ed3ab009e2ff41328333514a78549
Reviewed-on: https://gerrit.instructure.com/43341
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>