Fix #18586: rewrite model_package.ModelPackageTests.test_models_packages

Ticket https://code.djangoproject.com/ticket/18586
Oh, thank you for quick feedback!

Do you mean we should split this test into more parts or oracle related case should be extracted? 
For example, I see there is a possibility to extract test conditionally, like: `@unittest.skipUnless(connection.vendor == 'oracle', "Test only for Oracle")`