Test: fix draft test for emacs23

emacs24 and emacs23 have different secure tag defaults: in particular,
mml-secure-message-sign only signs the part on emacs23 but the whole
message on emacs24. This difference makes one of the draft tests fail
(which causes a cascade of later failures) on emacs23. It seems that
travis uses emacs23 so it is useful to fix this.

We do this by forcing the whole message to be signed in either case --
the code snippet is extracted from mml-secure-message-sign on emacs24.