Add deprecated element definition for Drake's unit testing.

When writing unit tests to exercise SDFormat deprecation notices, we can't
rely on any _actually_ deprecated elements, because they will disappear in
a future upstream release. Therefore, we inject a Drake-specific element
definition that will always be available.

--- src/parser.cc.orig
+++ src/parser.cc
@@ -379,7 +379,12 @@
   std::string xmldata = SDF::EmbeddedSpec("root.sdf", false);
   auto xmlDoc = makeSdfDoc();
   xmlDoc.Parse(xmldata.c_str());
-  return initDoc(_errors, _sdf, &xmlDoc, _config);
+  bool result = initDoc(_errors, _sdf, &xmlDoc, _config);
+  ElementPtr drake_testing_element(new Element);
+  drake_testing_element->SetName("_drake_deprecation_unit_test_element");
+  drake_testing_element->SetRequired("-1" /* deprecated */);
+  _sdf->Root()->AddElementDescription(drake_testing_element);
+  return result;
 }

 //////////////////////////////////////////////////
