function writeReferenceFrame(referencePath) {
  document.write(
      '<iframe id="reference" name="reference" ' +
      'src="' + referencePath + '"></iframe>');
}
Uses document.write to add an iFrame to the page with the reference path in
the src attribute. Used for loading an html file containing reference
structures to test against into the page. Should be called within the body of
the jsunit test page.
@param {string} referencePath A path to a reference HTML file.