title: Unit test for Kotlin lambda callback
body: <p>Let's say we have the following function to test</p>
  
  <pre><code>fun loadData(dataId: Long, completion: (JsonElement?, Exception?) -&gt; Unit) {
      underlayingApi.post(url = "some/rest/url",
              completion = { rawResult, exception -&gt;
                  val processedResult = processJson(rawResult)
                  completion(processedResult, exception)
              })
  }
  </code></pre>
  
  <p>It's clear to me how to mock, inject, stub and verify the calls to  <code>underlayingApi</code>.</p>
  
  <p><strong>How to verify the result returned via</strong> <code>completion(processedResult, exception)</code><strong>?</strong></p>