Well, I did determine both the shutoff valve and the shutoff lever easily stop the engine. But I decided to try doing it through the injectors anyway, since like whunter said, I could also inspect the injector nozzles. Everything went surprsingly smoothly until I actually went to do the test. As soon as the guage reaches 250 psi, the connector pops off. There's only one quick connector in the kit that fits the injector adapter, so I can't try another one to see if it's just a bum connector. That would be my guess. Naturally I got the cheap Harbor Freight tester. It worked fine with the glow plug adapter (it popped off once on that round but I assumed I had not seated it right, and it was fine every other time). Frustrating.
Also my injectors came out covered in oil, especially Nos. 1 and 2. Not an encouraging day.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)