Salesforce CategoryNode sObject in Apex Unit Tests

In Salesforce,DML operation is not allowed on CategoryNode sObject,even in Unit Test. Although the 《 Apex Code Developer’s Guide》 does not mention it clearly,that CategoryNode object can be accessed in Apex Tests, we have to prepare CategoryNode data first in Setup clicking Customize > Solution > Solution Categories.

The following is the sample code for CategoryNode and CategoryData sObject testing ,and it works for me.

Reference : CategoryNode API Reference  and   CategoryData API Reference