don't count test users in speedgrader graded count

Returns a separate JSON element consisting solely
of student with vanilla enrollments (not e.g.
student_view_enrollments) so that any test students
are excluded from grading counts.

fixes CNVS-3471

Test plan:
- Access an assignment whose class has a test
student enrolled.
- Go to SpeedGrader.
- Note the values of the  "x / y Graded" indicator
at the upper right of the page.
- Score the test user's assignment.
- The "x / y Graded" values should not have
changed.

Change-Id: I244f4463023bcf385aa609ed04e5b387ede1def9
Reviewed-on: https://gerrit.instructure.com/33684
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>