Improve xml parameter entity parsing, and add a new ERT test.

Improve xml parameter entity parsing, and add a new ERT test.

* test/automated/xml-parse-tests.el: New file.

* lisp/xml.el (xml--parse-buffer): New function.  Move most of
xml-parse-region here.
(xml-parse-region): Copy region into a temporary buffer, since
parameter entity substitution requires changing buffer contents.
Use xml--parse-buffer.
(xml-parse-file): Use xml--parse-buffer.
(xml-parse-dtd): Make parameter entity substitution work right.