Check before outputting needs in model-test

I submitted a similar pull request at ember-cli/ember-cli#2829. Coïncidentally I came across this in my own `ember-cli-mocha`-using application just now when generating a standalone test to [answer this StackOverflow question](https://stackoverflow.com/questions/27629728/how-can-i-unit-test-class-methods-in-ember-cli), so I figured I’d suggest it here too.

This is helpful for a model test generated by itself rather than at the same time as a model, because the lack of a `needs` in the locals passed in to the blueprint template results in the none of the template variables being substituted and consequently invalid Javascript, as shown [here](https://github.com/ember-cli/ember-cli/issues/2755#issuecomment-67851810) by @karlrl.

Perhaps you have an automated way to convert the QUnit blueprints and this is unnecessary, close away if so.

I didn’t see any tests to add to, but it’s the same change that passed the tests on `ember-cli`, and I confirmed that it worked in my own application.