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 behaviour.

also re-opened the `BaseProgressor` class instead of the User class, to send notifications after actions are performed, as that behaviour has been delegated-out from the User to ActionProcessors.
The indentation is a little off here.  I realise this is just moved from the original place, but this is a good opportunity to clean it up.