Add attname of fields that define a custom one to Model._shared_field_names and Model._translated_field_names
Normalize test use of data and fixtures.
- Clean up test project models.
- Rebuild fixture data declarations to use named tuples for clarity.
- Simplify declaration and inclusion of fixtures in test cases.
- Update the whole 215 tests so they use fixture names and not the values.
Clean up admin tests.
- Use fixtures instead of manual creation.
- Fix miscellaneous assumptions that may break on other DB engines.
Merge pull request #194 from spectras/fix/postgresql_tests

Rework the handling of fixtures in tests, makint it possible to fix PostgreSQL support.