Update spawn_test

launch/spawn_test.launch

 <?xml version="1.0"?>
 <!-- -->
 <launch>
+  <arg name="gazebo" default="true"/>
   <arg name="world_name" default="worlds/empty.world"/>
   <arg name="gui"   default="true"/>
 
-  <include file="$(find gazebo_ros)/launch/empty_world.launch">
-    <arg name="world_name" value="$(arg world_name)"/>
-    <arg name="gui" value="$(arg gui)"/>
-  </include>
+  <group if="$(arg gazebo)">
+    <include file="$(find gazebo_ros)/launch/empty_world.launch">
+      <arg name="world_name" value="$(arg world_name)"/>
+      <arg name="gui" value="$(arg gui)"/>
+    </include>
+  </group>
 
   <include file="$(find iri_object_description)/launch/spawn_object.launch">
     <arg name="name"   value="static_box"/>