spec: refactor some assignment selenium specs for draft state

refs #CNVS-15563

Change-Id: Ib9230a74045751b098e94078d0d9cccf792da464
Reviewed-on: https://gerrit.instructure.com/42302
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>