spec: rid assignment group spec of non DS tests

assignments_group_spec.rb is still testing non Draft State
cases. This commit will purge it of all non-DS tests
and refactor those tests to test in Draft State.

Change-Id: I07998249c0172d8569ccd46d4cff8391a570e71c
Reviewed-on: https://gerrit.instructure.com/43188
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matt Berns <mberns@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>