Is it possible to add a comment line in order to allow the user to add a comment to the quesition?
It should be inside the question "border" lines.

Which question type?

It is available for "List with comment" question type, for other question types you might have to create an additional open ended question and if you want it within the question border lines, you might have to move it via Javascript or something.

manual.limesurvey.org/Question_types#List_with_comment

Perhaps the comment field should be displayed for every question. A test and review field for tester?

Perhaps the comment field should be displayed for every question.

Who knows...