Add ert-font-lock as well as unit tests and testing resources.
* lisp/emacs-lisp/ert-font-lock.el: New library.
* test/lisp/emacs-lisp/ert-font-lock-resources/broken.js:
* test/lisp/emacs-lisp/ert-font-lock-resources/correct.js:
* test/lisp/emacs-lisp/ert-font-lock-tests.el: Unit tests.
(Bug#67460)