Improve ert-font-lock assertion parser (Bug#69714)

Improve ert-font-lock assertion parser (Bug#69714)

Fail on files with no assertions, parser now accepts multiple
carets per line and face lists:
* lisp/emacs-lisp/ert-font-lock.el: Assertion parser fix.
* test/lisp/emacs-lisp/ert-font-lock-resources/no-asserts.js:
* test/lisp/emacs-lisp/ert-font-lock-tests.el
(test-parse-comments--no-assertion-error)
(test-syntax-highlight-inline--caret-negated-wrong-face)
(test-macro-test--file-no-asserts): New test cases.
* doc/misc/ert.texi (Syntax Highlighting Tests): More syntax examples.