pub fn empty() -> anyhow::Result<Self> {
        let db_url = env::var("TEST_DATABASE_URL").context(
            "TEST_DATABASE_URL must be set. Normally, this is done by the 'zk' tool. \
            Make sure that you are running the tests with 'zk test rust' command or equivalent.",
        )?;
        Ok(Self(db_url.parse()?))
    }
Obtains the test database URL from the environment variable.
core/lib/db_connection/src/connection_pool.rs