Replace `include_once` with `require_once`
Replace `include_once` with `require_once`

Co-authored-by: Weston Ruter <334ec4b454fd8f7f815decf6ccc7eff3ade23140@google.com>
ampproject/amp-wp-qa-tester,ampproject/amp-wp-qa-tester,ampproject/amp-wp-qa-tester