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/test_utils.rs

@@ -624,7 +624,10 @@ impl TestContext {
     }
 
     pub async fn pop_sent_msg_opt(&self, timeout: Duration) -> Option<SentMessage<'_>> {
-        let rev_order = true;
+        let rev_order = !self
+            .get_config_bool(Config::PopSentMsgFromHead)
+            .await
+            .unwrap();
         self.pop_sent_msg_ex(rev_order, timeout).await
     }