Fix wordpress sample's "white screen of death" in CI environment

The [GAE Standard Wordpress sample](https://github.com/GoogleCloudPlatform/php-docs-samples/tree/master/appengine/standard/wordpress) has a ["white screen of death"](https://www.wpbeginner.com/wp-tutorials/how-to-fix-the-wordpress-white-screen-of-death/) in our CI environment.

For now, I've disabled those tests in #1336 - but we should fix the underlying issue.