I'd have the tests not show a simple line, but instead a complex checkerboard pattern you scan with a phone. The scan results would simultaneously test for 10

I'd have the tests not show a simple line, but instead a complex checkerboard pattern you scan with a phone. The scan results would simultaneously test for 100+ diseases, and be uploaded to some department of health server which would then decide if you specifically should be paid to stay home. That decision can be made based on fraud risk, but also on the benefit to society of that person not spreading that disease further. For example if a disease has already infected most of the town in the last few weeks, it makes no sense for someone to stay home because local immunity is already probably high and further spread unlikely. However the first case in a new town would 100% be worth staying home for to avoid infecting thousands of others.