Get the Pg engine using EngineTest.

There is one failing test still to fix; looks like an ordering bug (probably
need to tweak how I update `commited_at` in the SQLite tests).

After that, I need to:

* Retore the Pg _upate_ids tests (left commented-out).
* Rename EngineTest to DBIEngineTest.
* Get Pg on Role::DBIEngine.