If you want to delete test responses, Go to the Results tab and then to the Responses. Here you have the option of removing each of the responses individually or all the existing responses.
You can also delete all the results by going to Results tab -> Aggregated results and then click on three dots menu:
After clicking on the menu, the "Delete all responses" option will be visible. You can delete all completions, or only finished ones:
This action will remove all the results collected for this survey.