Really test that the jump to mapping dialog finds a site

- Before, this was only testing for the presence of the site host in
  any form, which would have matched on the error page's `<p>` tag and
  passed. We don't want it to match an error page, we want to know that
  the site has been found, so test for the site host in header text.
- Add a test for the "Unknown site" error page case.
- Remove www. in the test data to be consistent with the factory—I'd got confused with the thing I did for finding non-www sites.