Some cleanups in the embedded JDBC driver and its test

This is partly in preparation for using it more extensively in tests not directly related to Postgres.

An example is included: the only qp DT that uses pg today is switched to embedded with no ill effects.

There is still more to be done to get, e.g., `YamlTesterIT`, converted. Things like `EXPLAIN`, dealing with stray semicolons in YAML tests.