make dummy exception extend exception

tests/Doctrine/Tests/DBAL/RetryWrapperTest.php

@@ -130,18 +130,13 @@ class RetryCallableExample
     }
 }
 
-class DummyDriverException implements DriverException
+class DummyDriverException extends \Exception implements DriverException
 {
     public function getErrorCode()
     {
         return 'code';
     }
 
-    public function getMessage()
-    {
-        return 'message';
-    }
-
     public function getSQLState()
     {
         return 'state';