As part of the discussion on help-gnu-emacs[1], Stefan applied a simpler
patch in commit f478082f9f, only for font-lock-prepend-text-property
though.  Does it make sense to do the same in
font-lock-append-text-property?

As a learning experience with ERT, I wrote the attached test suite; with
the current master branch, only the prepend test passes, the other
fails.  Applying the same change in font-lock-append-text-property lets
both tests pass.