Fix #18586: rewrite model_package.ModelPackageTests.test_models_packages

Ticket https://code.djangoproject.com/ticket/18586
Oops, I didn't read the tests carefully. Actually, I think we could try to rewrite it so that it's not Oracle specific. I think something like this might work:
```python
from django.db import connection
from django.db.backends.utils import truncate_name

self.assertIn(
    truncate_name('model_package_article_publications', connection.ops.max_name_length()),
    Article._meta.get_field('publications').m2m_db_table()
)
```
Ignore my comment about it being a separate test.