Do not load real API client in template unit tests

The worker unit tests should never load the real API schema from a remote server, and instead use MockAPIClient.

You can setup the following line in the default fixture (in autouse):

monkeypatch.setattr(BaseWorker, "setup_api_client", lambda _: MockApiClient())

This will speedup all workers instantiations in unit tests by a 30x factor (and multiply that by number of tests...)