Refs #4189, adding Overlay screenshot tests:

- Make 'idsite' parameter in certain Overlay URLs 'idSite'.
- Make sure Overlay sidebar has width so screenshot test framework can find position.
- Add overlay test site so Overlay can be loaded in phantomjs.
- Remove createSuperUser call from TrackerTest (done in Fixture setUp).
- Make sure piwik.js link is created for screenshot tests & make sure the test environment JSON file is loaded after the database is setup.
- Modify logic that waits for images to load in screenshot testing framework's page renderer to wait for CSS image URLs.