Use deterministic port allocation in integration tests

Replace the bind-to-port-0-then-drop pattern in test port allocation
with an atomic counter starting at port 20000. The previous approach
had a TOCTOU race: the OS-assigned port was released when the probe
socket was dropped, allowing another test's node to grab it before
Node::start() could re-bind, causing sporadic InvalidSocketAddress
failures in CI.

An atomic counter guarantees unique ports across all parallel tests
within the process and is stable across node restarts.

AI tools were used in preparing this commit.

Author: joostjager

tests/common/mod.rs

@@ -14,6 +14,7 @@ use std::collections::{HashMap, HashSet};
 use std::env;
 use std::future::Future;
 use std::path::PathBuf;
+use std::sync::atomic::{AtomicU16, Ordering};
 use std::sync::{Arc, RwLock};
 use std::time::Duration;
 
@@ -268,17 +269,14 @@ pub(crate) fn random_storage_path() -> PathBuf {
 	temp_path
 }
 
-pub(crate) fn random_listening_addresses() -> Vec<SocketAddress> {
-	let num_addresses = 2;
-	let mut listening_addresses = HashSet::new();
-
-	while listening_addresses.len() < num_addresses {
-		let socket = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
-		let address: SocketAddress = socket.local_addr().unwrap().into();
-		listening_addresses.insert(address);
-	}
+static NEXT_PORT: AtomicU16 = AtomicU16::new(20000);
 
-	listening_addresses.into_iter().collect()
+pub(crate) fn random_listening_addresses() -> Vec<SocketAddress> {
+	let port = NEXT_PORT.fetch_add(2, Ordering::Relaxed);
+	vec![
+		SocketAddress::TcpIpV4 { addr: [127, 0, 0, 1], port },
+		SocketAddress::TcpIpV4 { addr: [127, 0, 0, 1], port: port + 1 },
+	]
 }
 
 pub(crate) fn random_node_alias() -> Option<NodeAlias> {