Look into using ZookeeperTestingServer to test ZooCache

[keith-turner]

In the following comment an experimental IT was written to test ZooCache performance.

#5040 (comment)_

While that performance test would not be suitable for an IT, it may be good to write an IT that test ZooCache correctness in a similar way to that test. There is a fairly extensive zoo cache unit test that uses mocking. It would be better to move a subset of those test to use a real zookeeper. That unit test assumes that when a node has zero children in zookeeper that null is returned which is not the case see #5047. The ZookeeperTestingServer is really fast so running the test would not take longer, but the test would be more accurate than mocking.