test: Adding non-maildir tags does not move message from new to cur

Some MUA's like mutt show the difference between "new" emails living in maildir
directory new/, and "old" emails living in maildir directory cur/. However
notmuch tag unconditionally moves selected messages from new/ to cur/, even if
no maildir synchronized tag is changed.

While maildir specification forbids messages with tags living in new/, there is
no need to move messages to cur/ when no maildir synchronized tag is changed.
Thus notmuch can remain transparent with respect to other MUA's.

[ Edited commit log to better describe the intended changes, and tag the
  test as broken until the actual changes are implemented -- Louis Rilling ]

Signed-off-by: Louis Rilling <l.rilling@av7.net>
[ Converted to use test_subtest_known_broken, David Bremner ]