title: How to use testharness for flink timer with 3 inputs
body: <p>I wanted to test the flink timers using testHarness, I can do it when I have one input key using <code>KeyedOneInputStreamOperatorTestHarness</code>.
  But I wanted to test the process function with 3 input key.</p>
  <p>Example:</p>
  <pre><code>class Sample: KeyedProcessFunction&lt;String, Tuple4&lt;String, input1?, input2?, input3?&gt;, Tuple2&lt;String, sampleOutput?&gt;&gt;
  </code></pre>
  <p>I am unable to use the <code>testHarness</code> for process function with more than 2 input keys?</p>
  <p>Any suggestions?
  Thanks!</p>