Test requirements often arise after system design is complete. Engineers need time to examine the system, theorize failure modes, and design the tests. Also time to test the system, find some unexpected failure mode, and then design yet more tests.
Would it be so hard to add a diagnostic interface? That’s all we’re talking about here. It seems like you’re making the problem more difficult than it is. And they’d had to have known ahead of time if they wanted to test voltage curves of motors.
Your answer is good in the general case, but for the anecdote, the design was clearly bad.
