Scaffold test fixtures

[guzman-raphael]

Some tests to consider:

• https://github.com/guzman-raphael/orcapod/tree/2d2c97f2ce38fde089a1be89a66a6d5b8db961fc/tests
• https://github.com/Synicix/orcapod/blob/d71adbe3e349d8e0e295001da8e1c9ca882056d4/tests/model.rs