Implement tests for submissions_by_grader_for_index (#1088)

* feat: Implement test for filter by assessment title

* feat: Implement random submission status in test seed

* feat: Implement test for filter by submission status

* feat: Implement random graded status in test seed

* feat: Implement test for filter by notFullyGraded

* feat: Improve test seed and make tests pure

* feat: Update seed to include second group and avenger

* feat: Implement tests for filtering by group

* feat: Implement tests for filter by group name

* feat: Implement tests for filter by student name

* feat: Implement tests for filter by student username

* feat: Implement tests for filter by manually graded config

* refactor: Remove whitespace and move asserts to the bottom

* chore: Remove unused variables in test seed

* refactor: Remove unnecessary code

Author: GabrielCWT