function Test (title, fn) {
  if (!isString(title)) {
    throw new Error('Test `title` should be a "string" but "' + typeof title + '" was given instead.');
  }
  Runnable.call(this, title, fn);
  this.pending = !fn;
  this.type = 'test';
}
Initialize a new `Test` with the given `title` and callback `fn`.

@api private
@param {String} title
@param {Function} fn