PrefixedLocalStorageTest = function () {
  PrefixedLocalStorage.call(this);
  this.prefix = `${document.location.pathname}-${Math.random()}-${Date.now()}-`;
}
**************************************************************************
Use in a testharnessjs test to generate a new key prefix.
async_test(t => {
  var prefixedStorage = new PrefixedLocalStorageTest();
  t.add_cleanup(() => prefixedStorage.cleanup());
  /...
});