Simple test case to ensure that transaction-isolation is properly set.

case 1) (for all 4 types of isolation), set isolation, create a new connection and check whether
it returns the connection with expected isolation-level.

case 2) (for all 4 types of isolation) create surplus connections in the pool (they will be created
with a particular isolation) and modify it to an isolation other than expected. getConnection()
 again must return with guaranteed isolation