notmuch-test: Test "notmuch reply" with a reply-to-munged mail

This is a test for the recently added feature where we detect that the
reply-to address already exists in the To: or Cc: header so will
already be replied to. In this case we want to include the From:
address in our reply, (where, otherwise we would use the Reply-To
address *instead* of the address in the From header).