Reworked AbstractXPathTest.

This base class (which provides common functionality for tests of several
implementation classes of the XPath expression engine) now operates on
ImmutableNode objects which can be accessed by a NodeHandler object.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1565791 13f79535-47bb-0310-9956-ffa450edef68