private void renderGroup(XMLWriter writer, HitGroup hit) {
        writer.openTag(GROUP);
        renderHitAttributes(writer, hit);
        writer.closeStartTag();
    }
Creates an XML renderer using a custom executor.
Using a custom executor is useful for tests to avoid creating new threads for each renderer registry.
container-search/src/main/java/com/yahoo/search/rendering/XmlRenderer.java