Deprecate adding child nodes to attributes.

This test was ported from Perl. The behaviour has changed in libxml 2.13.0,
but we're pushing the boundaries of valid tree-structure anyway.

lib/LibXML/Attr.rakumod

@@ -123,6 +123,10 @@ method is-valid($elem) { self.ownerDocument.is-valid($elem, self) }
     parse time).
 =end pod
 
+# Adding child nodes to an attribute node is technically allowed,
+# but makes no sense
+method addChild(|) is DEPRECATED("LibXML::Element for node parenting") { nextsame }
+
 #| DOM level-2 method NYI
 method specified { die X::NYI.new }
 

t/20extras.t

@@ -42,6 +42,8 @@ subtest 'cloneNode', {
     is $doc.Str, $clone.Str, "unbind of document element";
 }
 
+skip "attribute child nodes is deprecated";
+#`( # depreacted
 subtest 'attribute child nodes' => {
     plan 3;
     my LibXML::Document:D $doc .= parse: :file<samples/dtd.xml>;
@@ -53,3 +55,4 @@ subtest 'attribute child nodes' => {
     $elem.setAttributeNode($att);
     is $elem.Str, '<Test att="xxx&foo;"/>';
 }
+)