If you have access to your site's SQL server, you could run the following query:
DELETE FROM mdl_grade_grades WHERE userid = [#userid];
Just replace [#userid] by the id of the test user. Be aware that this is irreversible, you want to make sure you put the right userid in there or you will lose data forever and ever. Please don't run this on a production server unless you really know what you're doing.
Remove Grades for User
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