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
@david-binda 

> What if someone is using constant( 'WP_CONTENT_DIR' ) or is using sprintf or something? That might produce false positives

It's a pretty straightforward string. No reason for people to get overly complicated with it. fwiw, I don't think I've ever seen someone use anything other than `WP_CONTENT_DIR . '/themes/vip/plugins/vip-init.php'` because they just copy/paste from our documentation.