Fix modules[moduleId] error when running JS tests

Running LMS Karma tests was causing the following error:

  TypeError: modules[moduleId] is undefined
  at /edx/app/edxapp/edx-platform/lms/static/course_experience/js/spec/Enrollment_spec.js:20

The root issue was sinon, as documented here:
  https://github.com/webpack/webpack/issues/304

I tried multiple fixes suggested from that issue until I found one that
worked: https://github.com/webpack/webpack/issues/304#issuecomment-272150177