Deleted old test

- Tests from now on, are placed inside the file being tested

Signed-off-by: DanielFath <daniel.fath7@gmail.com>
Changed name of CDATANode and CommentNode to fit the style of project

Renamed:
- CDATANode => XmlCDATA
- CommendNode => XmlComment
Added a to_str() method to XmlDoc

- This method serializes the given XML document into a human readable form