test: Add emacs test for messages with very long subjects

Emacs search does not show messages with very long subjects. The
main notmuch view shows unread messages but nothing in the search
results. Thus you can not remove unread tag from the emacs
interface.

The test uses 4018 character long subject. This is the minimal
length to trigger the bug. I think the subject length should be
increased to a much higher value once the bug is fixed.

The test is currently failing.

The code is borrowed from the "Message with .. in Message-Id:"
test. Looks like this may be a common case for testing emacs
interface bugs with particular messages and should be moved to a
test-lib function.

Regards,
  Dmitry