Add explanation and small optimizations to LanguageTest

@singingwolfboy or @sarina 
Right now UniqueCourseTest is a subclass of WebAppTest and additionally sets up unique course info, in preparation for programmatically creating a new course. The language tests don't need that. I'm guessing that whoever created the language tests copied/pasted using another test as a template. 

Not the hugest deal, but since setUp for the LanguageTest class supers the setUp of its parent I'd like to prevent proliferation of this unnecessary overhead, especially since who know what will be later added there.