As of Emacs 29.1, In-Reply-To is in the default value for
message-hidden-headers. We actually want to see that in the test
suite, so remove it again. To future proof the tests, fix a default
value for message-hidden-headers specifically for the test suite.
;; environments
(setq mm-text-html-renderer 'html2text)
+
+;; Set our own default for message-hidden-headers, to avoid tests
+;; breaking when the Emacs default changes.
+(setq message-hidden-headers
+ '("^References:" "^Face:" "^X-Face:" "^X-Draft-From:"))