[Maintenance]: Add the ability to unit test with celery executor

[tjeerddie]

Description

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.

Tasks

• Add new celery tests to test/celery_tests
• Make use of pytest-celery to startup/configure a Celery worker
• Testcases:
  • Starting a workflow
  • Resuming a workflow
• Add a CI workflow in .github/workflows, using the same services as the other CI workflows (postgres/redis)

[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