Setup Mock Orderbook DBs for unit testing

[bh2smith]

In #3, we introduce some tests that are actually "integration tests", however, if we want to unit test this without relying on external sources, we should prepare two DBs (barn and orderbook) -- both populated with necessary seed data.

This will be similar to what we have done here:

https://github.com/cowprotocol/solver-rewards/tree/main/tests/db

[bh2smith]

This issue belongs in solver-rewards