Fix memory leak in wxXmlNode::operator=().

We must delete all children and attributes in the node being overwritten and
not just the first one of each.

Add a unit test exercising this code to be able to check that valgrind doesn't
report memory leak any more after the fix.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73990 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775