* This allows the tests to add their own FLX files but still use consistent fonts.
* The test fonts are only embedded on the desktop test shells. The option is not available on mobile platforms.
* Right now, all fonts will resolve to the test font. If we want tests to be able to use the fonts they embed in FLX files but use the test font for platform fallbacks, we will need to add font selector fallbacks. I can do this in an another patch. So far, there are no users of this functionality.