The intention with test data is to populate the fields on the checkout form when it is viewed by a non-member. This makes it easier to process transactions and create test member accounts. If you were viewing the page as an admin or as a logged-in member, you would not see the test data.
If the checkout page is in production and people are actively using it to make purchases, then you should disable test data. If you don't want test data to pre-populate the form fields, you will need to disable it, and only enable it when you wish to test. In order to disable the test data, go to Payment Settings > Test Data and then deselect the checkbox next to 'Use Test Data'.
For more information about using test data, you can review the video here: Speed Up Testing by Using Test Data