I agree; JSONML is a quasi-standard we have adopted, which is mainly useful for converting arbitrary XML to JSON. If you can decide how your XML format looks like, I would recommend the standard conversion format:
declare option output:method "json";
<json type="array" objects='_'>{
for $user in collection("saveresult")//user[_id="1f2cda8f-a18a-44ba-8d17-73626d472306"]
return
<testResults>
<testId>{$user/test/_id}</testId>
<grade>{$user/user_info/user_grade}</grade>
</testResults>
}</json>