Updates regex for checking if vip-init is present and adds test-theme to test/data

and adds VIPInitCheck related tests

work toward: https://github.com/Automattic/vip-scanner/issues/49
@joshbetz  happy to update the regex - just say so more confidently :wink: 

What if someone is using ``constant( 'WP_CONTENT_DIR' )`` or is using ``sprintf`` or something? That might produce false positives