Captive Portal test

This involves checking DNS resolution, comparing HTTP status codes content, and serial numbers for Start of Authority DNS records.

This test needs to be specified: https://trac.torproject.org/projects/tor/wiki/doc/OONI/Tests/CaptivePortal.

There is an implementation of this inside of isis' branch: https://github.com/isislovecruft/ooni-probe/tree/captiveportal. This needs to be reviewed and ported to the new OONI model.