Rewrite incremental tests to follow codebase patterns

Completely rewrite incremental serialization tests to match the
established patterns used in file_test.py and other serialization tests:

- Add pytest fixtures (hasher_factory, file_serializer) for reusability
- Use existing model fixtures from conftest.py (sample_model_folder)
- Use test_support helpers (extract_digests_from_manifest, get_first_file)
- Follow arrange-act-assert pattern consistently
- Simplify assertions using helper functions
- Remove all hardcoded fake digest values
- Tests are now ~230 lines shorter and more maintainable

All 7 tests pass and follow the same style as the rest of the test suite.

Signed-off-by: Emrick Donadei <emrick.donadei@gmail.com>

Author: edonadei