test(client): strengthen assertions for session cleanup after peer closure

Update tests to ensure no packets at all are sent following a peer-initiated closure, rather than just checking for the absence of the close flag.

crates/irtt-cli/src/shared/client/session.rs

@@ -288,11 +288,11 @@ mod tests {
             socket
                 .set_read_timeout(Some(Duration::from_millis(250)))
                 .unwrap();
-            while let Some((packet, _)) = recv_request_timeout(&socket) {
-                assert_eq!(
-                    packet[3] & flags::FLAG_CLOSE,
-                    0,
-                    "session cleanup must not send a close after peer close"
+            if let Some((packet, _)) = recv_request_timeout(&socket) {
+                panic!(
+                    "session cleanup must not send any packet after peer close; flags={:?} len={}",
+                    packet.get(3).copied(),
+                    packet.len()
                 );
             }
         });