xsd10.rng: correct wildcard attribute handling

The previous solution hacked around an issue with xml:lang and runs
afoul of default/inherited ns for the specific case of excluding
attributes with no namespace.  See documentation at openAttrs for
discussion.

Note this essentially reverts 0ac9030c9 now that I better understand how
this RELAX NG thing works.
runchecks: extend tests to cover PyXB schemas