enable test for failOnMissing

We've been using this fork for a little while now and I'd like to see it incorporated upstream.

To address the need for testing, I made a tweak so 'grunt test' will always set the 'force' grunt option, so that it doesn't fail and actually falls through to run the tests.

I also modified the task so that if failOnMissing is true, it doesn't actually output the file, and added a test to ensure that fact.