Provide an aggregate tests runner. See #4583

Provide an aggregate tests runner.
See: #4583
http://codeception.com/docs/08-Customization#One-Runner-for-Multiple-Applications
different constants would break consistency in the scenario of:
* https://github.com/yiisoft/yii2/issues/4583#issuecomment-51032645 

I dont want to check for different TEST_ENTRY_URL_FRONTEND TEST_ENTRY_URL_BACKEND in submodules, as only 1 TEST_ENTRY_URL for 1 @app is required.