Update Db.md

The documentation states that setting "cleanup" option to true, reloads the dump "after" each test. In my mind this would mean after a test is run, the database is left in a consistent state. In actual fact the code actually restores the dump "before" each test. This means that after a test is run, the database is in has the test changes. This difference is important if one is having more tests run (maybe a different suite) after the current suite which might not be using the Db module.