test: Add broken test for tag synchronization on files delivered to new/

Currently, notmuch new only synchronizes maildir flags to tags for
files that have an "info" part.  However, in maildir, new mail doesn't
gain the info part until it moves from new/ to cur/.  Hence, even
though mail in new/ doesn't have an info part, it is still a maildir
message and thus has maildir flags (though none of them set).

The most visible effect of not synchronizing maildir flags for
messages in new/ is that newly delivered messages don't get the unread
tag (unless it is assigned by some other mechanism, like new.tags).

This patch does *not* modify the test for messages in cur/ that do not
have an "info" part.  Unlike a message in new/, a message in cur/
without an info part is no longer a maildir message, and thus
shouldn't be subject to maildir flag synchronization.