I am not sure if you can delete the assignment - I think it falls under the same as with regular students.
You can look here for more information (I found out that you can actually remove the test student which I did not know about). However, if you remove the test student that may remove the assignment as well - not sure if it will come back when the test student comes back.
Good evening, @sabrams ...
I would agree with @bowmanr. I do not believe you can delete submitted assignments from the "Test Student" role. However, @bowmanr is correct that you could delete the "Test Student" altogether. To do that, you would go to your course "Settings" page, and then navigate to your "Sections" tab. On that tab, you should see a listing of all students in your course...including the "Test Student"...which probably has a long string of letters/numbers near it. Hover you mouse over the "Test Student", and hopefully you'll see a red "x". If you click this, then it would completely remove the "Test Student" from your course. But, the next time you use the "Test Student" role, it would re-add "Test Student" back to your Gradebook, and you'd have to repeat the process over again.
Another option for you, if you want to keep the "Test Student" role in your course, is to just reset the "Test Student". There are instructions in this Guide on how to do that: How do I view a course as a test student using Stu... - Canvas Community. Refer to the "Reset Student" section.
I hope this extra bit of information will be of help to you. Sing out if you have any other questions about this...thanks!