Run top-level test groups concurrently
The DB integration test suite now runs the groups concurrently: * Alonzo-era * Babbage-era * Conway-era * QSM (QuickCheck State Machine) Concurrent tests cannot share a database, so each group has its own copy. Grouping at the top level keeps the databases/threads manageable. Also, add the required databases in the nix `preCheck` hook and GitHub CI workflows.