Verify file size and position in testGetCurrentLogSize.

This is pre-PR for ZOOKEEPER-4714.

In ZOOKEEPER-4714, we maintain fileSize and filePosition ourselves and we want our values to match the original values. Therefore, we added checks for fileSize and filePosition in our tests. After adding the checks, we used a new method to retrieve fileSize and filePosition in ZOOKEEPER-4714 and tested whether the tests can still pass.