Document how to selectively run tests
Document how to selectively run tests

This assumes consistent naming described in the preceding commit. Still
doesn't answer how we make people aware of all our flags, per #21.

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