Support database interaction in tests

We want our integration tests to run against a test database.  This
allows us to create more specific tests (e.g. predict ID values) in
addition to making sure we don't interact with real data.

I decided to use the PostgreSQL schema support instead of requiring
a separate `TEST_DATABASE_URL`.  This means there is less setup for
a dev environment but also makes it easier for us to drop / create the
schema as part of the tests.