Do you get "testable/verifiable specifications, with a test plan" if you buy a $100k Mercedes Benz, and because you don't, is Mercedes Benz a scam?
Come on, try to be realistic. If it doesn't work, can't be constructed, is missing parts etc., that's clear enough to file a dispute. Not having a test plan is just normal.
You are taking the 'test plan' too literal. Read: 'installation manual' with steps to do simple diagnostics. Some simple diagnostic steps are part of every car manual.
Cars are one example where yes, any worthwhile automaker will have a standard test/inspection plan.
This reminds me of a true story. The first car I bought (a Toyota) was used, from a wholesale dealer. Before the final sale, I asked if I could take the car to a Toyota dealership to have it inspected. They said sure, no big deal, so I did so and the dealership ran their 160-point inspection, for which I was given a report. No issues turned up and I bought the car from the wholesale dealer.
Fast forward about 6 months, and my check engine light came on. I took it to my local Toyota dealer (different from the one who did the inspection). They told me the CEL was for a leak in the fuel system, which they discovered via a simple smoke test. The rep also told me that the technician who diagnosed the issue visually confirmed pinholes in the fuel filler assembly. Estimated cost of parts and labor was around $1.5K.
It was a bit hard to believe that holes would show up within 6 months. So I contacted the Toyota dealer which had originally done the inspection and explained the situation. They told me to come down and they'd check it out. After waiting for 30 minutes the owner of the dealership brought me into the shop where they had the smoke test set up. They said smoke was leaking from the gas cap, not the fuel neck. He pulled out some steel wool, which he used to scrub off rust on the top of the fuel filler neck, put the gas cap back on, and the smoke disappeated. I thanked him, shook hands, and never had the issue again. Never went back to my local dealer.
The point I'm making is that testable specifications are your best tool for avoiding scams. I would never buy something as expensive (and dangerous) as a car, even a ten year old beater, without such an assurance.
Of course, in the case that the seller is beyond the reach of your court system, even testing and verification doesn't mean much.