test: MDN on pre-message has effect if received before sending full message (#8004)

Actually, the problem in #8004 is that a pre-message doesn't "want MDN" if it has no text. Anyway,
the added test reproduces the bug.

src/mimeparser.rs

@@ -2498,6 +2498,8 @@ async fn handle_mdn(
     let Some((msg_id, chat_id, has_mdns, is_dup)) = context
         .sql
         .query_row_optional(
+            // MDN on a pre-message references the post-message, see `receive_imf`. So we can't tell
+            // which one was seen, but this is on purpose.
             "SELECT
                 m.id AS msg_id,
                 c.id AS chat_id,