Improve testing coverage after PR

# Problem

This [PR](https://github.com/code-corps/code-corps-ember/pull/1463#issuecomment-340201852) resulted in a substantial drop in test coverage (~1%) due to refactoring of imports. This seemingly shouldn't have an impact on testing coverage; however, the current thought is the imports from before this PR were picked up as covered by coveralls when in fact the actual logic was not covered. Here is an [example](https://coveralls.io/builds/13934987/source?filename=app%2Fmixins%2Fonboarding-controller.js).

Inizia con l'esempio di Coveralls per app/mixins/onboarding-controller.js e confronta la relativa coverage con le modifiche di PR 1463, in particolare il refactoring degli import. Individua la logica segnalata come coperta senza test, quindi aggiungi la coverage per quel comportamento e verifica che il report complessivo non diminuisca più dopo il refactoring.

Scritto dal modello di indicizzazione a partire dal testo della issue.