Graphviz: add missing support for circular dependencies

This support was actually added by removing a superfluous check that was
too eager in cmLinkItemGraphVisitor.cxx.

Extended the existing Graphviz tests to include this particular case,
and re-generated the expected output files.

Fixes: #20720

Source/cmLinkItemGraphVisitor.cxx

@@ -24,10 +24,6 @@ void cmLinkItemGraphVisitor::VisitItem(cmLinkItem const& item)
 void cmLinkItemGraphVisitor::VisitLinks(cmLinkItem const& item,
                                         cmLinkItem const& rootItem)
 {
-  if (this->LinkVisited(item, rootItem)) {
-    return;
-  }
-
   if (item.Target == nullptr) {
     return;
   }