Tests should clear old assessments before running.

[Luro02]

You can delete semi-automatic assessments through artemis here:

{artemis_url}/course-management/{course_id}/exams/{exams_id}/exercise-groups/{exercise_group}/programming-exercises/{exercise_id}/participations/{participation_id}
For example:
https://artemis.abc.com/4/exams/4/exercise-groups/5/programming-exercises/44/participations/162

The endpoint is likely not yet implemented in artemis4j.