Rewrite custos authnz unit tests

[jdavcs]

The custos authnz unit tests use mocked representations of SQLAlchemy objects to test the galaxy.authnz.custos_authnz module. In doing so, they break the code base when the module under test refernces (directly or indirectly) a SQLAlchemy object that used in a way that is not mocked out in this test module.

The test module needs a complete rewrite. SQLAlchemy objects should not be mocked.

For context, see #20893