Detect driver in tests based on configured driver name

[morozov]

Q	A
Type	improvement
BC Break	no

If a driver is wrapped in a decorator, then $connection->getDriver() will return the decorator, not the actual driver. It is better to drive the test logic based on the test configuration rather than based on introspecting the connection.

Additionally:

1. Enable testing unique constraint violation handling on IBM DB2 and SQL Server (see Basic exception handling for SQL Server #4928, Basic exception handling for IBM DB2 #4929).
2. Reuse the logic from PDO\ConnectionTest to cover all drivers. Otherwise, the test would require another assertion against the driver class.