test-org-footnote: Fix a broken test

* testing/lisp/test-org-footnote.el: Fix failing
  `test-org-footnote/normalize-outside-org'.

testing/lisp/test-org-footnote.el

@@ -212,7 +212,8 @@ Footnotes:
   ;; 3. As an exception, in `message-mode' buffer, if a signature is
   ;;    present, insert footnotes before it.
   (let ((org-footnote-tag-for-non-org-mode-files nil)
-        (message-signature-separator "^-- $"))
+        (message-cite-prefix-regexp "\\([ 	]*[_.[:word:]]+>+\\|[ 	]*[]>|]\\)+")
+	(message-signature-separator "^-- $"))
     (with-temp-buffer
       (insert "Body[fn::def]
 --