Add tests for the Course class

[lizTheDeveloper]

Add tests to the Course class, in lib/course.js.

• Add 10 real examples from the curriculum as fixtures
• Ensure that all yaml-derived properties are populated properly
• Ensure the slug is set correctly
• Ensure that therender function can reproduce what was parsed, even for unsupported yaml fields
• Ensure that the workouts and sections in a course are ordered by the sections field on the file that is parsed, that Course.workouts field has workouts in their absolute order, and that the Course.sections field has each section from the sections field on the readme, and that each section has workouts in the same order as the slugs from the sections field.