Refactor reload_openzaak_fixture to use TestContextDecorator

[SilviaAmAm]

Suggested by Victorien. I had tried, but for some reasons the tests started hanging in CI. I didn't have the time to investigate it further.

Decorator: https://github.com/maykinmedia/open-archiefbeheer/blob/feature/165-destroy-cases/backend/src/openarchiefbeheer/utils/utils_decorators.py

Docs: https://github.com/django/django/blob/cf03aa4e94625971852a09e869f7ee7c328b573f/django/test/utils.py#L384

Also fix that in development mode it checks if we are running from cassettes