Improve tests for actual destruction #1081

Closes #1078

Note: we briefly discussed adding an e2e test yesterday, but it seems not really straightforward. It appears to not trigger destruction at all, even with a CELERY_TASK_ALWAYS_EAGER=True override. Also on the feature e2e test for list destruction, a lot of assertions seem to be missing with a future TODO:

open-archiefbeheer/backend/src/openarchiefbeheer/destruction/tests/e2e/features/test_feature_list_destroy.py

Lines 34 to 36 in e8e3e82

	await self.then.path_should_be(page, "/destruction-lists")
	# TODO: Seems to rely on open zaak (mock?)
	# await self.then.list_should_have_status(page, destruction_list, ListStatus.deleted)

I can spend more time on it if you want, but I'm not sure how long it will take. The unit test I have added also covers the current bug

I do at some point want a more thorough test for the actual destruction, but like you said it's not trivial. I also want a WAITING_PERIOD setting of 0 to start destruction immediately, maybe this an be combined.

Originally posted by @svenvandescheur in #1079 (comment)