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
@Ragazzo Maybe you are right in case of redistributable framework such as yii. And developers should aggregate their tests by a third-party software or a command line script. But the approach with globals suits well to my case.
Maybe we should define a constant with unique name for each application. E.g.: TEST_ENTRY_URL_FRONTEND TEST_ENTRY_URL_BACKEND.