Refactors tests in preperation for the multi-experiment refactor

Changes creation of tracks and intervention points in tests to use a constructor in tests/common.
Renames exceptions to refer to experiments rather than courses.
Introduces `assertOkay` function in place of `assertEqual(response.status_code, 200)`.
Fixes indentation in tests.
Deletes tests for CourseSettings as this model is getting changed substantially.

Adds several fields and methods to models to spoof new behavior that will be introduced in the multi-experiment code.