More flexibility to choose which tests are run

[jason-math]

Problem: With large hidden test cases, the grader can be quite slow. This can be problematic since students have to wait a long time to receive feedback.

Solution: A way to toggle which tests are run so hidden test cases don't hinder student submission feedback
Some possible ideas:

1. Allow tests to be run_after_due_date and only be run once submissions close
2. Leave it up to the user and allow for docker env variables to be modified without reuploading/updating the autograder so they can toggle the environment variables to change how the grader runs.