Run schema tests against govuk-content-schemas repo

Changes the tests that validate publishing schemas so that they reference the external alphagov/govuk-content-schemas repository. The location of the local checkout of this repo can be overridden via an environment variable, but it defaults to a sibling directory of Whitehall. The Jenkins script is updated to automatically check it out to /tmp before running tests.

Additionally a rake task `test:publishing_schemas` is provided to run just the publishing presenter tests, for use in the branch build of govuk-content-schemas.