Fix PG DbTest::testHaveAndSeeInDatabase

- The test was failing because the value for lastInsertId when
using PG was always 0.
- As per http://www.php.net/manual/en/pdo.lastinsertid.php,
  PDO::lastInsertId for PG requires the sequence name. This fix
  guesses the sequence name based on the table. The test now passes.