remove test student scores from assignment data

fixes CNVS-10570
fixes CNVS-11104

test plan:
  - as a teacher enter in scores for students and a test student
  - make test student grade the hightest (not tied)
    - on the grades page, in assingment stats, the test student score shouldnt show up
  - make test student grade the lowest (not tied)
    - on the grades page, in assingment stats, the test student score shouldnt show up
  - on the grades page, test student score shouldnt change the average

Change-Id: I9916a999cf7db4872b09d03e8e14883503025d96
Reviewed-on: https://gerrit.instructure.com/29091
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Matthew Irish <mirish@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>