I could imagine some sort of signed-submission system. The test device generates a QR code you can print or screenshot, and the inspector can scan that and it'

I could imagine some sort of signed-submission system. The test device generates a QR code you can print or screenshot, and the inspector can scan that and it's either a device-signed message or a link to a repository which would say "this test was performed by machine 123456 at 2020-04-23 10:00:20, result negative" If someone aims to defraud the system by having someone else take the test for them, or by forging the submission system, there's probably some fraud-related legal recourse.