test: Add test that emacs interface actually sends mail.

test: Add test that emacs interface actually sends mail.

Rather than *reall* sending mail here, we instead have a new test
program, smtp-dummy which implements (a small piece of) the
server-side SMTP protocol and saves a mail message to the filename
provided. This gives us reasonable test coverage of a large chunk of
the notmuch+emacs code base (down to talking to an SMTP server with
the final mail contents).