Refactor namespace-related tests in xml::node test suite
Refactor namespace-related tests in xml::node test suite

Factor out NamespaceTest helper fixture to make adding more
namespace-related tests simpler.

No real changes.

vadz/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp,vslavik/xmlwrapp