mod: incorporate suggestions
- rename requireApp() to mockApplication()
- always destroy app on tearDown()
  - eliminates need for constant YII_DESTROY_APP_ON_TEARDOWN
  - mockApplication() becomes a lot easier. Destroying app on each tearDown
    means creating it on every call is fine. No more checking if it already
    exists and if it has been created from the same config.
- \yii::$app should have been \Yii::$app