test: add known broken test for reply to message with multiple Cc headers

test: add known broken test for reply to message with multiple Cc headers

As Daniel Kahn Gillmor <dkg@fifthhorseman.net> reports in
id:87d1ngv95p.fsf@alice.fifthhorseman.net, notmuch show combines
multiple Cc: fields into one, while notmuch reply does not. While such
messages are in violation of RFC 5322, it would be reasonable to
expect notmuch to be consistent. Add a known broken test to document
this expectation.

This also starts a new "broken" corpus for messages which are broken.

Details:

The original message is formatted using the message printing in
notmuch-show.c. For Cc:, it uses g_mime_message_get_recipients(),
which apparently combines all Cc: fields into one internally.

The addresses in the reply headers, OTOH, are based on headers queried
through libnotmuch. It boils down to g_mime_object_get_header() in
lib/message-file.c, which returns only the first occurence of header.