For more information, please see here.Android Developers
Sample Ad Units
Google provides Test ID’s for testing your ads. Use those ID’s aslong your game is in developing. When it’s live, change the ID’s to your own. The ID’s also can be found at the official admob documentation available here.
|Ad format||Sample ad unit ID|
Just use the Ad ID’s above in your Construct 2 project. Then the test ads will be shown instead of live ads.