test: known broken test for emacs display of message/rfc822 parts

test: known broken test for emacs display of message/rfc822 parts

There are at least 3 bugs present.

1) notmuch-show-insert-part-message/rfc822 assumes that message/rfc822
parts will have a ":content" property, but that turns out not to be
the case.

2) something deep in gnus wants gnus-newsgroup-charset, but that is
defined in gnus-sum, which is not loaded by default.

3) If gnus-sum is loaded, then the display of the message/rfc822 part
succeeds, but the buffer gets put into gnus-article-mode, which means
that, inter alia, notmuch text properties and keybindings get wiped.