Test: tools: validity tests should not depend on libxml2 version

This particular change increases robustness against this particular
change (in error.c) applicable for v2.9.0+:
https://git.gnome.org/browse/libxml2/commit/?id=97fa5b3c8f3fe6ff79123ab43ed6b846fd1dd6dd

Also, reflect that change in the expected results.