removed test param 'appClass' as it does not make sense to configure
application class for all tests, some explicitly need webapp where
some might explicitly need console app.
- 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