The new JavaScript API allows you to listen to the event that indicates that the report is done loading:
https://github.com/Microsoft/PowerBI-JavaScript/wiki/Handling-Events
I'm not sure whether you could use this directly in your test. A different approach could be to listen for the 'loaded' event in your application code and then make some changes to the DOM in your application (something that can be detected by Selenium) that flags the report as 'finished loading'.
Just an idea, hope this helps 