This question has a fundamental error in that tests are most often performed by other agencies than the physician or medical group, therefore the physician cannot be the supplier of these tests. Similarly, services are also most performed by other agencies for which the physician cannot be considered the supplier. One way this scenario can hold as a generality is if the physician is taking "kickbacks," or pay-backs for sending business to other agencies.
Regardless of the truthful reflection of reality in the question scenario, ordering tests and/or services for patients for the purpose of financial gain and little or no other reason ("just to be sure ...") is a violation of the medical oaths taken by doctors.
Doing significant research on the medical problem or condition being treated may provide a reasonable way to know if tests and/or services are being inappropriately or appropriately ordered.
Further Reading