What would you put in the "light" QR code ? If you put the date of the test, then the organizer can access it. If you don't, then they can't check whether your test is recent enough to be authorized to attend their event.
Like the original author states, than can't be done without a trusted 3rd party ( say a token with a service telling you whether the pass is valid at current date)
why not simply having 2 QR codes, one full, one light ... ?