Tested for issue #1418: Added tests calculating the median and average for assignments.

There are three tests: one testing for an even number of submissions (where the bug originated from), another testing an odd number of submissions, and finally, a test for checking submissions with varied marks (which also happens to have an even number of submissions). This copy is without arkon94's changes, so the two tests with the even number of submissions fail, but when his change is merged they will pass. I felt these three cases were essential considering the bug, let me know if anyone feels there are other significant cases that should be tested.