spec_helper: Ensure empty 'users' DB table.
spec_helper: Roll back DB after every example run.
Add Preserves::SQL.connection.

We need to use the same connection for the tests and the transaction
surrounding the tests, or else the tests won't be inside the
transaction that we're rolling back after each test.

Also simplify the transaction/rollback code in spec_helper.
Make Preserves::Mapper#query actually hit the DB.