Avoid using cma-cases slugs in publication_log_spec
Avoid using cma-cases slugs in publication_log_spec

CMA Cases are now published by specialist-publisher[1]. Updating the
slugs makes the tests a bit more realistic.

I've used the 'guidance' prefix as it's hardcoded in the
`ManualBuilder` as the prefix for all manuals.

[1]: https://github.com/alphagov/specialist-publisher

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher