More Tests for Grading

So sorry for the delay, I really did not expect my last week to be that busy... Please review all the new tests for grading functionality and let me know what you think. Four remaining tests will come later tonight.

By the way, I'm really having some hard time getting more users enrolled in the course. I tried the usual way(using post request), but it seems the email enrollment mechanism is a bit tricky to handle during testing. I also tried hack the database to manually add participation record, but it does not seems to solve the problem, I still get 403 when try to start a quiz session. Did I miss something here? Any better solution? There is no doubt that adding more users will help our test, but I'm not sure if it is really necessary.