How do I setup Karma to run my backend unit tests (written with Mocha)? If I add my backend test script to the
files = 
You don't. Karma is only for testing browser-based code. If you have a project with mocha tests on the backend and karma/mocha on the front end, try editing your package.json under scripts to set test to:
mocha -R spec && karma run karma.con
npm test returns true, you'll know it's safe to commit or deploy.