fix(性能测试): 修复性能测试输出jmx时自定义变量位置不对的问题

--bug=1010212 --user=刘瑞斌 【接口测试】github#10431，在性能测试中添加自定义参数后执行，检查结果报错 https://www.tapd.cn/55049933/s/1103322

Closes #10431

backend/src/main/java/io/metersphere/performance/parse/xml/reader/JmeterDocumentParser.java

@@ -393,7 +393,7 @@ public class JmeterDocumentParser implements EngineSourceParser {
       </Arguments>
          */
 
-        Element element = ele.addElement(ARGUMENTS);
+        Element element = hashTree.addElement(ARGUMENTS);
         element.addAttribute("guiclass", "ArgumentsPanel");
         element.addAttribute("testclass", "Arguments");
         element.addAttribute("testname", "User Defined Variables");