Sadly the "temp" in with-temp-file refers to a buffer, not a 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):
Delete temporary files at end.