Add automated test for "Add a favicon for the wiki"

The manual test is here: https://test.xwiki.org/xwiki/bin/view/Administration%20Tests/Add%20a%20favicon%20for%20the%20wiki.

The test is not fully automatable because at the moment we don't know how to assess that there's a favicon in a browser tab. Also, the test needs to be run on all browsers.