The Praefect smoke test job sets up the Postgres service on its own,
even though we have the `postgres_definition` which would already go
most of the way of setting it up correctly.
Convert the test to use the shared Postgres service definition instead
to avoid code duplication. The only thing that needs to change for the
definition is that we didn't have a database name defined at all, which
is added as part of this commit.
