Fix test_etcp_simple_traffic for new tun_if API

tests/test_etcp_simple_traffic.c

@@ -294,7 +294,7 @@ int main() {
     }
 
     // Initialize connections and register sockets regardless of TUN state
-    if (server_instance->tun.fd < 0) {
+    if (!server_instance->tun) {
         printf("ℹ️  Server TUN disabled - initializing connections only\n");
     }
 
@@ -323,7 +323,7 @@ int main() {
     }
     
     // Initialize connections and register sockets regardless of TUN state
-    if (client_instance->tun.fd < 0) {
+    if (!client_instance->tun) {
         printf("ℹ️  Client TUN disabled - initializing connections only\n");
     }