I wrote this test code...

<?php /*

Composr
Copyright (c) ocProducts, 2004-2016

See text/EN/licence.txt for full licencing information.

*/

/**
* @license http://opensource.org/licenses/cpal_1.0 Common Public Attribution License
* @copyright ocProducts Ltd
* @package testing_platform
*/

/**
* Composr test case class (unit testing).
*/
class schemadotorg_test_set extends cms_test_case
{
public function setUp()
{
require_code('lorem');

parent::setUp();
}

public function testScreens()
{
$tpl = render_screen_preview('AUTHOR_SCREEN.tpl', 'authors', 'tpl_preview__author_screen');
$this->do_validation($tpl->evaluate());
}

protected function do_validation($data)
{
$url = 'https://search.google.com/structured-data/testing-tool/u/0/validate';
$post_params = array('html' => $data);
$result = http_download_file($url, null, true, false, 'Composr', $post_params);
}

public function tearDown()
{
parent::tearDown();
}
}

... but I can't find a validator we can easily use as an API. Google's returns a load of JavaScript.