stub script json file writes

[davidsbailey]

some tests for course / unit group were generating files with names like unit3.script_json, on the test machine and when run locally:

this PR fixes the instances of this which I could find.

Testing story

running the following no longer generates stray script_json files:

Dave-MBP:~/src/cdo/dashboard (stub-script-json-file-writes)$ bundle exec spring testunit test/models/unit_group_test.rb test/controllers/courses_controller_test.rb

[dmcavoy]

Thank you!

These all look like tests I created so TIL that add File.stubs will prevent this. Thanks!