Hi @YuJiang1208 ...
Even though when you use the "Student View" to view your course as a student, it does put a "Test Student" student in your course "Grades" page. However, you'll also notice that it does not add the "Test Student" role to your "People" page. The "Test Student" is not an actual student...even though you can view parts of the course as if you were a student using this functionality. So, unfortunately, it is correct that you would not be able to add the "Test Student" role to any Groups that you have/had set up within your course.
Sorry that I don't have better news for you, but I hope this helps to answer your question.