I don't understand what the point of the fine is. I'd imagine any person who cares enough about Covid to voluntarily take a test would also care enough to register the outcome. The kinds of people who would deliberately keep a positive result a secret are not going to be self-administering tests. The answer can't be that it's intended to incentivise otherwise compliant people who may be too busy to declare the result, because we know tens of thousands of infections are happening daily due to a lack of tests. This wouldn't do shit as far as improving data is concerned.
And yea you're right, it's unenforceable and everyone knows it.
There is literally no reasonable public interest angle to the fine. Which means we have to assume it's for show; a budget-neutral way for the government to look like it's doing something without the fuss of actually having to do anything.