Test rework

Rework unit tests to minimize the number that depend on external assets. (Fixes an issue where failed functional tests prevent everything else from running)

With this code, everything passes on branch `gh-pages`, and all but 11 pass on `core`. fix swcarpentry/lesson-template#54
 
I wasn't thrilled with the duplication either. If you want to create an enhancement issue, perhaps we can track cleanup of the unit tests for later as a separate task?