Expose tests through public API in src

[oxinabox]

If a package loads AbstractDifferentiation to implement the interface,
it should also be able to test it.
We have code for testing it,
but that code lives in test/

[gdalle]

Related issues:

• Add a function to test the interface is correctly implemented #12
• Expectations for implementations of the interface #13

[oxinabox]

closing as duplicate of #12