Clean up 5 expected failures

https://redmine.teklia.com/issues/7025

• arkindex.documents.tests.test_corpus_authorized_users is removed entirely, because the tests are duplicates. arkindex.documents.tests.test_corpus already asserts on the authorized_users attribute.
• I had fixed two expected failures when working on !2393 (merged), and had also fixed two unrelated ones in arkindex.ponos.tests.test_artifacts_api, which are now here.