fix(测试跟踪): 导入xmind用例失败

--bug=1018422 --user=陈建星 【测试跟踪】导入xmind用例失败 https://www.tapd.cn/55049933/s/1267829

test-track/backend/src/main/java/io/metersphere/xmind/parser/XmindZen.java

@@ -25,7 +25,7 @@ public class XmindZen {
             transferNotes(rootTopic);
             Map children = rootTopic.get("children");
             recursionChildren(children);
-            contents.add(jsonObject.toString());
+            contents.add(JSON.toJSONString(jsonObject));
         }
         return contents;
     }