title: Check if checkbox is checked with jest/enzyme
body: <p>How do I test if several checkboxes are checked or not?</p>
  
  <pre><code>render()
      ...
          &lt;React.Fragment&gt;
                &lt;div
                  className='foo'&gt;
                  &lt;label&gt;
                    &lt;input
                      className='checkbox'
                      name='bar'
                      type='checkbox'
                      checked={this.props.checked}
                      onChange={() =&gt; { } }
                    /&gt;
                  &lt;/label&gt;
                &lt;/div&gt;
          &lt;/React.Fragment&gt;
      ...
  </code></pre>
  
  <p>I tried</p>
  
  <pre><code>it('checks all checkboxes', () =&gt; {
      const component = shallow(
          &lt;Foo
              ... /&gt;
      )
  
      expect(component
          .find('input[type="checkbox"][checked="checked"]'))
          .toHaveLength(content.length);
  });
  </code></pre>
  
  <p>and</p>
  
  <pre><code>component
      .find({ type: 'checkbox' })
      .forEach(node =&gt; {
          expect(node
              .props
              .checked)
              .toEqual(true);
      });
  </code></pre>
  
  <p>or </p>
  
  <pre><code>component
      .find('input')
      .forEach(node =&gt; {
          expect(node
              .props
              .checked)
              .toEqual(true);
      });
  </code></pre>
  
  <p>and</p>
  
  <pre><code>component
      .find('.checkbox')
      .forEach(node =&gt; {
          expect(node
              .props
              .checked)
              .toEqual(true);
      });
  </code></pre>
  
  <p>The last three give me <code>undefined</code> while the first one returns a massive object (<code>{Symbol(enzyme.__root__): {Symbol(enzyme.__root__): [Circular], Symbol(enzyme.__unrendered__): &lt;Foo....</code></p>