Use httptest.Server.URL in helper tests
Use httptest.Server.URL in helper tests

Rather than constructing our own `http://localhost` URL. This is in
preparation for enabling TLS backends, which will mean that we can no longer
assume that the scheme is HTTP.

http://golang.org/pkg/net/http/httptest/#Server

mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,alphagov/cdn-acceptance-tests,mikepea/cdn-acceptance-tests,mikepea/cdn-acceptance-tests