Add the ability to unit test with celery executor

[tjeerddie]

It would be great if we can test the application with celery as executor, so we can be sure it works corretly and can prevent issues like #271

Now when setting the EXECUTOR=celery in your .env, tests break with Celery has not been initialised yet error.

[Mark90]

The recently released pytest-celery plugin is probably useful here

https://github.com/celery/pytest-celery/releases/tag/v1.0.0 https://docs.celeryq.dev/projects/pytest-celery/en/latest/getting-started/first-steps.html