fix(gossipsub): fix check for fanout peer inclusion in test_join

The check for fanout peer inclusion in `test_join` does not check anything since `new_peers` is always empty.

https://github.com/libp2p/rust-libp2p/blob/12b785e94ede1e763dd041a107d3a00d5135a213/protocols/gossipsub/src/behaviour/tests.rs#L611

I assume the intention was to fill the `new_peers` with the fanout peers. In this MR I do just that.

Pull-Request: #3628.

protocols/gossipsub/src/behaviour/tests.rs

@@ -580,7 +580,7 @@ fn test_join() {
     // add 3 random peers to the fanout[topic1]
     gs.fanout
         .insert(topic_hashes[1].clone(), Default::default());
-    let new_peers: Vec<PeerId> = vec![];
+    let mut new_peers: Vec<PeerId> = vec![];
     for _ in 0..3 {
         let random_peer = PeerId::random();
         // inform the behaviour of a new peer
@@ -598,6 +598,7 @@ fn test_join() {
         // add the new peer to the fanout
         let fanout_peers = gs.fanout.get_mut(&topic_hashes[1]).unwrap();
         fanout_peers.insert(random_peer);
+        new_peers.push(random_peer);
     }
 
     // subscribe to topic1