test: net-GH-5504 child env needs NODE_COMMON_PORT

Author: tjfontaine

test/simple/test-net-GH-5504.js

@@ -73,7 +73,12 @@ function parent() {
   var serverExited = false;
   var clientExited = false;
   var serverListened = false;
-  var opt = { env: { NODE_DEBUG: 'net' } };
+  var opt = {
+    env: {
+      NODE_DEBUG: 'net',
+      NODE_COMMON_PORT: process.env.NODE_COMMON_PORT,
+    }
+  };
 
   process.on('exit', function() {
     assert(serverExited);