Expand valid/invalid tests to check v4/v6 separately

It would be nice to show the valid/invalid URL fetch results with v4 and v6 answers separated. Mainly to confirm both are working or not working. Of course this could also show that no v6 is seen by the browser - which is not the point of this site.

Start by locating the existing valid/invalid URL fetch tests and the code that records their results. Check how IPv4 and IPv6 answers are currently represented, then separate the results so both can be confirmed independently. Done means the tests report distinct v4 and v6 outcomes, including when no v6 is seen by the browser.

Written by the indexing model from the issue text.