It wouldn't even have to be a test though. It could be as simple as something you sign in to online, with a limited number of items where you watch some kind of informational video and tick a box that says, yes I have watched this video. It would be an annual opportunity to introduce any changes to the Highway code, or emphasise things that have been identified as being commonly misunderstood or ignored.
The worst that could happen is that some people play the video, don't pay any attention to it and tick the boxes. It would still be better than nothing. The idea that there'd be no meaningful benefit as the result of some people finding out some things they didn't know about, things that ignorance of could well have an impact on safety - that just seems implausible to me. I've no problem with someone doing the proper studies to provide the "proper causitive evidence" but instead of faffing around avoiding changing the current rubbish setup, why not just implement something quite simple with a high likelihood of benefit that as far as I can see could not do any harm?
I have some thoughts about why that doesn't happen, and I don't think it's due to lack of proper causitive evidence.