Make all PG-requiring tests feature-gated

[nyurik]

We need to simplify our testing process, and that means that cargo test should pass without any external dependencies, i.e. without requiring PG to be running, or some env var to be set.

I think PostgreSQL tests are the only complex ones, so lets mark them all as requiring some test-pg feature (name?). Note that cargo test --all-features should still include the postgreSQL tests, and thus require some connection string to be set. This will be done by CI, but will make development simpler.