Fix hi-lock test and add new test for case-fold (bug#40337)

Fix hi-lock test and add new test for case-fold (bug#40337)

* lisp/hi-lock.el (hi-lock--regexps-at-point): Handle font-lock faces.
(hi-lock-unface-buffer): Simplify default value handling.
(hi-lock-set-pattern): Add either lighter or regexp to
hi-lock-interactive-lighters.
(hi-lock-set-pattern): Put overlay prop hi-lock-overlay-regexp to
either lighter or regexp.

* test/lisp/hi-lock-tests.el (hi-lock-bug26666): Use "b" instead of "a".
(hi-lock-case-fold): New test.