message: tests: Adjust move coverage tests
  Large test wasn't testing shared parent condition properly - add one
  more level of hierarchy so that it works as expected.
diff: @@ -1715,16 +1715,20 @@ TEST(document_move_large)
  {
  xml_document* doc = new xml_document();
  + xml_node dn = doc->append_child(STR("node"));
  +
  for (int i = 0; i < 3000; ++i)
  - doc->append_child(STR("node"));
  + dn.append_child(STR("child"));
  xml_document other = std::move(*doc);
  delete doc;
  + xml_node on = other.child(STR("node"));
  +
  for (int i = 0; i < 3000; ++i)
  - CHECK(other.remove_child(other.first_child()));
  + CHECK(on.remove_child(on.first_child()));
  - CHECK(!other.first_child());
  + CHECK(!on.first_child());
  }
  TEST_XML(document_move_buffer, "<node1/><node2/>")
  