How to mock global variable which holds React Native native module function

In App.js I have some thing like

Class App extends Component {
 constructor(props){
  super(props)
  global.test = NativeModules.TestClass
 }
}

And in Test class I am using it like

Class Test extends Component {
 constructor(props){
  super(props)
  global.test.testFunction("Testing")
 }
}

So how to mock global.test.testFunction for the above class