PrefixedMessageResource = function () {
  PrefixedMessage.call(this);
  // Check URL querystring for prefix to use
  var regex = new RegExp(`[?&]${this.param}(=([^&#]*)|&|#|$)`),
    results = regex.exec(document.location.href);
  if (results && results[2]) {
    this.prefix = results[2];
  }
}
Instantiate in a test support script to use a "prefix" generated by a
PrefixedMessageTest in a controlling test file. It will look for
the prefix in a URL param (see also PrefixedMessage#url)