For discussion, see thread starting at
https://lists.gnu.org/archive/html/emacs-devel/2018-05/msg00222.html.
* lisp/custom.el: (custom-available-themes): Use directory-files
instead of performing arbitrary wildcard expansion in file names.
(custom-theme--load-path): Document return value.
* test/lisp/custom-tests.el: New file.
(custom-theme--load-path): New test.
