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 file is a bit of a dumping ground w/o any clear organization. I didn't want to try to figure out where the tests should go: just separate the ones which req'd importing from those which didn't.