Mock clients for testing

[cuixq]

We now rely on real data for testing and this causes us updating snapshots quite often.

Considering the work for interfaces and clients are done, we can implement mock clients and use them for testing.

[G-Rath]

fwiw that's a nice side effect of #1603 for the local databases, though when @another-rex and I discussed it yesterday we weren't a fan of how it meant we would no longer be testing against the real world advisories and infrastructure...