Fix undigest-tests on MS-Windows

Fix undigest-tests on MS-Windows

* lisp/emacs-lisp/ert-x.el (ert-with-temp-file): Accept a new
keyword argument :coding CODING to use as the encoding when
writing initial text to the temporary file.

* test/lisp/mail/undigest-tests.el (rmail-undigest-test-rfc934-digest)
(rmail-undigest-test-rfc1153-digest-strict)
(rmail-undigest-test-rfc1153-less-strict-digest)
(rmail-undigest-test-rfc1153-sloppy-digest)
(rmail-undigest-test-rfc1521-mime-digest)
(rmail-undigest-test-multipart-mixed-digest): Force the temporary
mbox files to have Unix-style EOL format.