[feature-scrub] Updated to verify via css+xpath

Previously, I was searching for Strings.

This was ok for a bit, but its not going to work when I start passing
things through nokogiri to handle emoji via html-pipeline. We're going
to start failing because we're looking for:

<img src="blah/png">

But we actually have the equivalent:

<img src='blah/png'>
Enhanced .features to verify via xpath and css

Merge commit '8697656' into upcoming

* commit '8697656':
  [feature-scrub] Updated to verify via css+xpath