contributing: add guidance on unit tests
Description
Try to decrease uncertainty around whether unit tests are welcome in the project.
Try to establish some common ground, before later proposing more pro-unit-test changes.
Changes proposed in this merge request:
- State that exercising things via the cli is to be preferred.
- State that unit tests are good where they are robust to unrelated changes made to internals.
Edited by Angelos Evripiotis