Fix some glitches in recent directory-files-* changes

Fix some glitches in recent directory-files-* changes

* doc/lispref/files.texi (Contents of Directories):
Fix description of directory-files, directory-empty-p and
directory-files-and-attributes.

* etc/NEWS: Fix entry for directory-files-and-attributes.  Fix typos.

* lisp/dired.el (directory-empty-p): Move function from here ...

* lisp/files.el (directory-empty-p): ... to here.

* lisp/net/ange-ftp.el (ange-ftp-directory-files): Call `nreverse' later.

* lisp/net/tramp.el (tramp-handle-directory-files):
* lisp/net/tramp-adb.el
(tramp-adb-handle-directory-files-and-attributes): Do not call
`nreverse'.

* src/dired.c (Fdirectory_files)
(Fdirectory_files_and_attributes): Fix docstrings.

* test/src/dired-tests.el: Removed.  Tests moved to
test/lisp/dired-tests.el.

* test/lisp/dired-tests.el (dired-test-bug27899): Tag it :unstable.
(dired-test-directory-files)
(dired-test-directory-files-and-attributes): New tests.