title: Selecting value in Slider with Widget Test
body: <p>I have a screen that contains a series of Sliders where the user can select if they agree/disagree with something, for example:</p>
  
  <pre><code>Slider(
    key: Key(questionId),
    min: 0,
    max: 5,
    divisions: 5,
    value: _value,
    onChanged: (value) {
       setState(() {
         _value = value;
       });
    },
  ),
  </code></pre>
  
  <p>I am currently implementing a UI test using <code>testWidgets</code>, in which the user selects multiple values in Sliders and then get stored when the user clicks on a button, and I'd like to be able to select a value, for example, to swipe the slider to value '2'.</p>
  
  <p>How can I do that?</p>
  
  <p>The only idea I have is to use <code>tapAt</code> with the exact pixel coordinates where the value is in the slider but seems extremely complicated to do.</p>
  
  <p>For clarification I am asking about Widget Testing as described in: <a href="https://flutter.dev/docs/cookbook/testing/widget/introduction" rel="noreferrer">https://flutter.dev/docs/cookbook/testing/widget/introduction</a></p>