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.

I probably wouldn't make this change given the lack of improvement. These test classes are already confusing because they are named for how they are implemented rather than for what they are testing. I find that breaking this into two different classes only makes that worse. I'd like to see these tests moved out into the more focused test classes, but that isn't necessary for this PR.