Add tests to workout parser

[lizTheDeveloper]

Add tests to the Workout class in lib/workout.js.

• Test the parser with 10 real workouts from the curriculum
• Ensure all yaml properties are set
• Ensure that insights are ordered by the insights yaml field
• Ensure that Exercises are properly parsed
• Ensure that the slug is properly set for workouts
• Ensure that the .toJSON() property can create or update our mongoose workout models
• Ensure that the render method can reproduce what was parsed, even for unsupported yaml fields