Created a new helper class for tests related to node structures.

Some common functionality which is useful for multiple test class was
refactored into a separate test helper class.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1570342 13f79535-47bb-0310-9956-ffa450edef68
Reworked ConfigurationNodeVisitor interface.

A generic parameter is now used to define the nodes to be iterated over. A
NodeHandler object has to be provided for accessing information about the
nodes.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1570343 13f79535-47bb-0310-9956-ffa450edef68