fix viewing test student grades by teacher

fixes CNVS-12584

test plan:
- as a teacher, enter student view and then exit (to create the test
  student)
- go to gradebook and give them a grade on an assignment
- click on their name to see their individual grades page
- it should load
- when you click on the 'score details' you should get a message the
  grade stats are not available for the test student.

Change-Id: I39a3d24d606569372515ec903fedbdce1e3e7558
Reviewed-on: https://gerrit.instructure.com/35474
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>