It's not that the supernatural can not be tested for. Rather, it is the believers in the supernatural who will not let any test shake their faith in the existence of the supernatural.
It would be a lot different if religious believers could agree on one version of the TRVTH, but there are some 4,300 world religions and tens of thousands of different sects or branches, each with its own interpretations of its selected sacred books or revelations. Many of the claims are internally inconsistent and/or contradictory. That's no way to instill confidence that one out of thousands has the TRVTH.
Nor is there any empirical way to judge among all of the competing claims, as they all ultimately go back to "trust me" somewhere.
Science uses verifiability as one of its keystones. If it can't be reproduced by other scientists it is not likely to be accepted for long (think cold fusion).
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.