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.
Merge pull request #400 from alphagov/really_test_jump_to_mapping

Really test that the jump to mapping dialog finds a site