More work for refactoring add_model() to add_model_object(). Tests pass

lib/Slic3r/Test.pm

@@ -108,7 +108,7 @@ sub init_print {
     $model_name = [$model_name] if ref($model_name) ne 'ARRAY';
     for my $model (map model($_, %params), @$model_name) {
         $model->arrange_objects($config);
-        $print->add_model($model);
+        $print->add_model_object($_) for @{$model->objects};
     }
     $print->validate;