fix quoted sequence name

This is not really a schema manager test. Please remove this test class again and instead add a test in `Doctrine\Tests\DBAL\Platforms\OraclePlatformTest`. You can adopt [this one](https://github.com/deeky666/dbal/blob/DBAL-831/tests/Doctrine/Tests/DBAL/Platforms/OraclePlatformTest.php#L504-L538).