test: save buffer content to file instead of printing it in Emacs tests

[notmuch] / test / emacs.expected-output / notmuch-show-thread-with-hidden-messages

Jan Janak <jan@ryngle.com> (2009-11-17) (inbox unread)
 Jan Janak <jan@ryngle.com> (2009-11-17) (inbox)
 Carl Worth <cworth@cworth.org> (2009-11-18) (inbox unread)