On an Amazon server cluster somewhere on Earth! Sorry, could not resist the golden opportunity you provided.
The document submitted as the Test Student will stay associated with the Test Student and with the course until you reset the Test Student or delete it from your section.The submitted document can be found in SpeedGrader for that assignment - again, only until you reset the Test Student.
Canvas does not maintain a File area for the Test Student like it does for other users. To access the document, you would need to leave Student View, then as a teacher, access it through SpeedGrader.
Can I ask why you are asking this? It might help me better guide you.