[ravindra-RKB]

This PR introduces unit tests for the XMLIO module under io/xml/test/, which currently lacks dedicated Google Test coverage.

Changes:

• TXMLEngine unit tests:
  • Tests DOM construction, nested elements, string/integer attributes, and in-memory serialization/parsing (SaveSingleNode, ParseString).
  • Tests XML document creation, file output (SaveDoc), and file parsing (ParseFile).
• TXMLFile unit tests:
  • Tests writing and reading ROOT objects (TH1I, TObjString, TNamed) to/from XML files.
  • Tests directory hierarchies (mkdir, cd), subfolder navigation, and key retrieval.
  • Uses ROOT::TestSupport::FileRaii and std::unique_ptr to ensure clean resource management without leaving files on disk.
• Registers the test directory in io/xml/CMakeLists.txt and adds the gtest target in io/xml/test/CMakeLists.txt.

Checklist:

• Tested changes locally
• Follows ROOT coding style and formatted with clang-format
• Minimal and focused changes

[linev]

Not needed.
There are several tests for TXMLEngine and TXMLFile in
https://github.com/root-project/root/tree/master/roottest/root/io/xml