libTrafficLightsGUIPlugin lacks a library when using gazebo_ros

[robin-shaun]

Hello, ${Qt5Test_LIBRARIES} is needed in CMakeLists.txt, or the gazebo_ros cannot load libTrafficLightsGUIPlugin.

  target_link_libraries(${guiSrc}
                        ${GAZEBO_LIBRARIES}
                        ${Qt5Core_LIBRARIES}
                        ${Qt5Widgets_LIBRARIES} 
                        ${Qt5Test_LIBRARIES}
                        ${GAZEBO_LIBRARIES}
  )