[DOC] Teardown test callback should reset application

Relates to ember-qunit/issues#29

The testing guide recommend to reset the application at the setup module callback, however ember-qunit `moduleFor` method does some job before the `setup` is executed which could originate exceptions if the application has not yet reseted.

If the application is reseted at `teardown`, it will be in a clean state when the test routine is initiated.