emacs: test notmuch-show during message decryption

We did not have a test showing what message decryption looks like
within notmuch-emacs.  This change gives us a baseline for future work
on the notmuch-emacs interface.

This differs from previous revisions of this patch in that it should
be insensitive to the order in which the local filesystem readdir()s
the underlying maildir.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>