Don't consider test student submissions when determining course unpublishability

Fixes CNVS-14490

Test plan:
- Create a new course, and an assignment within
- Enter student view to generate the test student enrollment
- Submit the assignment as the test student
- As a teacher, give the student a score for the assignment
- With the test student's submission graded, the course should
still be unpublishable

Change-Id: Ib11fb7a2baa834ddf0a7f39a8ac12542539ab1e4
Reviewed-on: https://gerrit.instructure.com/39009
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>