This is the same fix as 2019-04-29 "Refrain from splicing anonymous
faces in text properties", which was only applied to
font-lock-prepend-text-property.
* lisp/font-lock.el (font-lock-append-text-property): Distinguish list
of faces from property list.
* test/lisp/font-lock-tests.el: New test suite.
(Bug#35476)