Unique IDs in Tests

[Thetwam]

We recently received a PR (#398) where we were setting a quiz group's course ID to self.id rather than self.course_id. This should have been caught by our tests, but wasn't since we set the id of most of our test objects to 1. It's entirely possible there are other similar issues hidden because we reuse ID numbers so frequently.

Convert our tests to use unique ID numbers between object types. (e.g. 5 for courses, 27 for quizzes, etc.)

This will likely be a fair bit of work, since we reuse the fixtures in many places.