Add tests to cover XSS via meta tag values

Hello! I wanted to take a stab at this since I don't have a ton of experience writing tests, so any and all feedback is welcome.

In order to verify that the tests do indeed fail if xss was is not being filtered I temporarily set \Drupal\Component\Utility\Html::escape() and Drupal\Component\Render\PlainTextOutput::renderFromHtml() to both have a method body of return (string) $text;

Thanks! Hopefully this at least a decent start and is not too far off base.

Here is an attempt at making this xss test ported to the D7 branch. To verify the tests would fail if input was not sanitized, I disabled check_plain temporarily.

Please let me know if I should make any changes.
Thank you!

Here is that xss test ported to the D7 branch again. It think it was failing before because I created the patch wrong and a new file was not being created when the patch was applied in the test.