notif/tests: check test state (#360)

Relates to #340

### Description
Added the check that the peer's `state` is `Closed`. This is the
expected state since:
- in the beginning we check that the peer starts with state
[Closed](https://github.com/paritytech/litep2p/blob/e0b7118ea2ca3e9f9beb9605f649d7e9dcdd24c4/src/protocol/notification/tests/substream_validation.rs#L275)
- then we check that it changes to `state`
[Validating](https://github.com/paritytech/litep2p/blob/e0b7118ea2ca3e9f9beb9605f649d7e9dcdd24c4/src/protocol/notification/tests/substream_validation.rs#L295)
- then we have a
[NegotiationError](https://github.com/paritytech/litep2p/blob/e0b7118ea2ca3e9f9beb9605f649d7e9dcdd24c4/src/protocol/notification/tests/substream_validation.rs#L325)
- so then we should check again that the peer's state is reset to
`Closed`.

### Additional Changes
Replaced also all instances of 
```
_ => panic!("invalid state for peer"),
``` 
with 
```
state => panic!("invalid state for peer: {state:?}"),
``` 
since it might be more helpful for debugging purposes. If not necessary,
I can revert these.

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>

Author: Imod7

src/protocol/notification/tests/substream_validation.rs

@@ -84,9 +84,9 @@ async fn substream_accepted() {
     // connect peer and verify it's in closed state
     notif.next_event().await;
 
-    match notif.peers.get(&peer).unwrap().state {
+    match &notif.peers.get(&peer).unwrap().state {
         PeerState::Closed { .. } => {}
-        _ => panic!("invalid state for peer"),
+        state => panic!("invalid state for peer: {state:?}"),
     }
 
     // open inbound substream and verify that peer state has changed to `Validating`
@@ -182,9 +182,9 @@ async fn substream_rejected() {
     // connect peer and verify it's in closed state
     notif.on_connection_established(peer).await.unwrap();
 
-    match notif.peers.get(&peer).unwrap().state {
+    match &notif.peers.get(&peer).unwrap().state {
         PeerState::Closed { .. } => {}
-        _ => panic!("invalid state for peer"),
+        state => panic!("invalid state for peer: {state:?}"),
     }
 
     // open inbound substream and verify that peer state has changed to `Validating`
@@ -271,9 +271,9 @@ async fn accept_fails_due_to_closed_substream() {
     // connect peer and verify it's in closed state
     notif.next_event().await;
 
-    match notif.peers.get(&peer).unwrap().state {
+    match &notif.peers.get(&peer).unwrap().state {
         PeerState::Closed { .. } => {}
-        _ => panic!("invalid state for peer"),
+        state => panic!("invalid state for peer: {state:?}"),
     }
 
     // open inbound substream and verify that peer state has changed to `InboundOpen`
@@ -329,7 +329,10 @@ async fn accept_fails_due_to_closed_substream() {
     }
     notif.on_handshake_event(peer, event).await;
 
-    // TODO: https://github.com/paritytech/litep2p/issues/340 check state
+    match &notif.peers.get(&peer).unwrap().state {
+        PeerState::Closed { .. } => {}
+        state => panic!("invalid state for peer: {state:?}"),
+    }
 }
 
 #[tokio::test]