The tests only test for antibodies, not specific diseases. If you have an infection of any kind your body produces antibodies so the test will come out positive. Koch's Postulates and the Rivers Criteria both specify that to determine the tocicity of any pathogen it must first be isolated. This makes viruses particularly difficult as they need a host to stay alive. The SARS-CoV-2 virus has never been isloated so it can't be tested for as of yet.