Add unit tests to font_fit.js

The original font_size_utils.js script listens to lazy-load events to update heading so that the headers newly inserted into the DOM are resized/centered.
Also, if the header is not visible at the time it is appended to the DOM, we trigger the lazy-load event manually in the app when it becomes visible so that the resizing/centering logic can work.

Now with the web components the first reason isn't required anymore. However it's still unclear to me whether the second use case of the lazy-load event is still needed or not. I prefer to keep these tests cases commented just in case we need them in the future.