test: fix test_content_add_validation() for swh.model 0.0.64
since content_add may now raise a ValueError (originating from the model).
Migrated from D2963 (view on Phabricator)
Staging instance, all changes can be removed at any time
since content_add may now raise a ValueError (originating from the model).
Migrated from D2963 (view on Phabricator)