Adding Fragment support for shallow .find and .findWhere

[madicap]

This fixes #1664 by altering childrenOfNode() in RSTTraversal to return (recursively if needed to accommodate nesting) the children of any node it encounters that is a Fragment instead of the Fragment element. It additionally updates matchAdjacentSiblings() and matchGeneralSibling() to make use of childrenOfNode() instead of looking directly at parent.rendered in order to make use of the new Fragment logic, which lives inside of the ReactSixteenAdapter's isFragment()

Consider the following example

const FragmentExample = () => (
  <div className="container">
    <React.Fragment>
      <React.Fragment>
        <span>A span</span>
      </React.Fragment>
    </React.Fragment>
  </div>
);

Calling const wrapper = mount(<FragmentExample />); will yield a wrapper with the following structure:

<FragmentExample>
  <div className="container">
    <span>
      A span
    </span>
  </div>
</FragmentExample>

and direct selectors like wrapper.find('.container > span') will behave as expected to find the span.

If we instead say const wrapper = shallow(<FragmentExample />), however, the wrapper has additional layers in the form of <Symbol(React.fragment)>:

<div className="container">
  <Symbol(React.fragment)>
    <Symbol(React.fragment)>
      <span>
        A span
      </span>
    </Symbol(React.fragment)>   
  </Symbol(React.fragment)>
</div>

These additional layers caused direct selectors to fail, since simply looking at the .rendered of the container div would show the symbol type node, and not a span, as expected.

Please let me know if I need any additional tests.

[ljharb]

This is excellent, thank you!