test: Drop test for propagating flag changes from one file to another

There's nothing in the current API documentation that would suggest
the behavior being tested here. Attempt to implement this could have
some nasty side effects, (such as notmuch_message_maildir_flags_to_tags
implicitly calling notmuch_message_tags_to_maildir_flags and maybe
even opening up some bad looping possibilities).

Much better to stick with what we have documented, which we believe will
actually be useful, (and easy enough to comprehend).