Write tests for external grading directory construction

[nwalters512]

The root cause of #1489 could have been caught if we had test about the lib/externalGraderCommon.js#buildDirectory function. That function is "pure" in that it just takes a bunch of objects and builds a directory on disk from them - it doesn't read from or write to the DB (although it does read some files from disk). Given that, it should be relatively easy to test in isolation.