Re-write tests to use test helpers for workflow

govuk_content_models introduced test helpers in 24.0.0
which provide a consistent way to test workflow behavior.

also re-opened the BaseProgressor class instead of the
User class, to send notifications after actions are
performed, as that behavior has been delegated-out from
the User to ActionProcessors.
Merge pull request #287 from alphagov/use-latest-govuk-content-models

Re-write tests to use test helpers for workflow