Feature/additional tests (part 2)

* adds tests for function assign_track_and_create_student, other controller methods, new model methods, and adds two more tests to cover all of experiment_pages
* note: this is Part 2 of feature/additional_tests.


How would I know that the one or both of the track weights don't already match what you have below?  It would read better to explicitly pass in the weight to the `create_test_track_weight` function.