Checkstyle does not check unit test classes 关注

When I run Checkstyle on the whole project, it will run on all other classes than the classes with unit tests.

Also, when I go to the CheckStyle-View, the run button is green and works on usual classes (opened in the editor), while it is greyed out on test classes.

I am using a checkstyle-configuration in an xml-file, that I have used and maintained in Eclipse before I switched to IntelliJ just now.