tests(proto): Gate PLPMTUD test

There is a should_panic test which depends on a debug assertion to
panic. This commit makes the test enabled only when debug assertions
are enabled. This fixes `cargo test --release`.

quinn-proto/src/connection/mtud.rs

@@ -731,6 +731,7 @@ mod tests {
         assert!(completed(&mtud));
     }
 
+    #[cfg(debug_assertions)]
     #[test]
     #[should_panic]
     fn mtu_discovery_with_peer_max_udp_payload_size_after_search_panics() {