Make ResourceLoaderFileModule#getAllStyleFiles include all skin styles

* Add the ResourceLoaderFileModule#getAllSkinStyleFiles method,
  which returns all of the skinStyles files for a given module
* The LessFileCompilationTest and checkLess.php script, which use
  the #getAllStyleFile method, now validate all LESS style files

Bug: 63343
Change-Id: Ib2eb5761919c648aea4ae58f8d0531799fe7982b