> What WiFi devices are regularly conducting speed tests without being asked?

ISP provided routers, at least Xfinity does. I've gotten emails from them (before I ripped out their equipment and put my own in) "Great news, you're getting more than your plan's promised speeds" with speedtest results in the email, because they ran speed tests at like 3AM.

I wouldn't be surprised if it's happening often across all the residential ISPs, most likely for marketing purposes.