Allow the caller to specify own face on suffix in annotation-function

Allow the caller to specify own face on suffix in annotation-function

* lisp/minibuffer.el (completion--insert-strings):
Don't add 'completions-annotations' face when the caller
specified own face in annotation-function.
Remove no-op code for 'unless prefix' branch.
(completion-metadata, completion-extra-properties):
Update docs of affixation-function.
Suggested by Clemens <clemera@posteo.net> (bug#45780)

* test/lisp/minibuffer-tests.el: Rename package name from
completion-tests.el to minibuffer-tests.el.
Add new test completion--insert-strings-faces.

* doc/lispref/minibuf.texi (Completion Variables)
(Programmed Completion): Update descriptions of
annotation-function and affixation-function.