New directory pruning algorithm breaks integration tests

[jmwski]

In 76c4cb8, java-manta's directory pruning algorithm was updated to throw an IllegalArgumentException if the pruneDepth parameter is less than -1, the value that tells the implementation to prune as many parent directories as possible. However, the integration tests rely on this algorithm working correctly for values of pruneDepth less than -1. We should update the tests and perhaps add one that verifies that an IllegalArgumentException is thrown when the value of pruneDepth is less than -1.