function fixtureWrapper(parentNode = document.createElement('div')) {
  document.body.appendChild(parentNode);
  cachedWrappers.push(parentNode);
  return parentNode;
}
Creates a wrapper as a direct child of `<body>` to put the tested element into.
Need to be in the DOM to test for example `connectedCallback()` on elements.

@param {Element} [parentNode]
@returns {Element} wrapping node