test: Fix port 0 bug in atomic counter initialization

fetch_update returns the previous value, so the first caller got port 0
instead of the random base. Use compare_exchange for one-time init
followed by fetch_add, which correctly returns the base port to the
first caller.

AI tools were used in preparing this commit.

Author: joostjager

tests/common/mod.rs

@@ -276,19 +276,17 @@ pub(crate) fn random_storage_path() -> PathBuf {
 
 static NEXT_PORT: AtomicU16 = AtomicU16::new(0);
 
+fn init_base_port() {
+	// Initialize once with a random base port. compare_exchange ensures only one thread wins.
+	let base = rng().random_range(10000..50000u16);
+	let _ = NEXT_PORT.compare_exchange(0, base, Ordering::Relaxed, Ordering::Relaxed);
+}
+
 pub(crate) fn random_listening_addresses() -> Vec<SocketAddress> {
 	// Use an atomic counter to avoid intra-process collisions between parallel tests.
 	// The base port is randomized once per process to avoid inter-process collisions.
-	let port = NEXT_PORT
-		.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |p| {
-			if p == 0 {
-				let base = rng().random_range(10000..50000u16);
-				Some(base + 2)
-			} else {
-				Some(p + 2)
-			}
-		})
-		.unwrap_or_else(|p| p);
+	init_base_port();
+	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 },