make --fortesting flag generate a binary that points to localhost as the bootstrap base url

[erwinmombay]

currently if we host the code on a cloud offering like heroku (for testing) any embedded code is broken because it will try to fetch an integration file that is not yet hosted

https://github.com/ampproject/amphtml/blob/master/src/3p-frame.js#L203

[erwinmombay]

fyi @cramforce

[erwinmombay]

closed by #5890