This adds a test for https://github.com/chatmail/core/pull/7032/.
The crash happened if you received a message that has the from contact
also in the "To: " and "Chat-Group-Member-Fpr: " headers. Not sure how
it happened that such a message was created; I managed to create one in
a test, but needed to access some internals of MimeFactory for that.
I then saved the email that is sent by Alice into the test-data
directory, and then made a test that Bob doesn't crash if he receives
this email. And as a sanity-check, also test that Bob marks Fiona as
verified after receiving this email.
