Would you be supportive of an "adblock test page" that literally just reports if the adblocker is working correctly, rather than how good it is? Like maybe an EICAR-like rule that is added to EasyList that matches an element on that page?

There is. If you are using uBO Lite, enable the "uBlock test filters" list, wait a bit and click on the home page button at the right. It will lead to https://ublockorigin.github.io/uBOL-home/tests/test-filters....