Performance enhancements for unit tests

in test_contentstore, don't load the xml course if not necessary

@andy-armstrong plz review or assign to someone in T&L to review
@jzoldak fyi, you're welcome to review or propose someone on your team to review.

This looks like more changes than it is. I moved the tests which I thought really needed to do the import (i.e., were testing importing) into one class and the ones that didn't into another. For the ones that didn't, I created a stub course via the factory.