Ensure all tests chain to superclass setUp/tearDown methods

Our main test case classes do some important work in their setUp and tearDown methods which shouldn't be skipped.

In particular, BaseDjangoTestCase.setUp resets the context, and skipping that can have very confusing consequences: for example, running debusine.server.views.tests.test_artifacts.UploadFileViewTests.test_put_file_user_token followed by debusine.server.views.tests.test_artifacts.UploadFileViewTests.test_put_private_workspace_authorized failed with insert or update on table "db_groupauditlog" violates foreign key constraint "db_groupauditlog_actor_id_eaef5816_fk_db_user_id", because a group audit log entry was created with a reference to a context user left over from the previous test.