tests: use random port in listen test

closes #5162

Author: rluvaton

test/app.listen.js

@@ -6,7 +6,7 @@ describe('app.listen()', function(){
   it('should wrap with an HTTP server', function(done){
     var app = express();
 
-    var server = app.listen(9999, function(){
+    var server = app.listen(0, function () {
       server.close(done)
     });
   })