title: mock method calling callback function sinon
body: <p>How do you mock an outer method calling a callback using sinon?
  Example given the following code, getText should return 'a string' as response in the callback function</p>
  
  <pre><code>sinon.stub(a, 'getText').returns('a string')
  let cb = function(err, response) {
     console.log(response)
  }
  a.getText('abc', cb)
  </code></pre>
  
  <p>and it should produce the output 'a string' since it calls callback function cb but there is no output</p>