Make the test as specific to your physical experience as possible.
I have a lot of uphill experience (first as a climber for fifteen years, then as a skimo racer for six). So my body “knows” how to go uphill.
In contrast, I only ever run out of necessity. I don’t do it for its own sake and I don’t have a significant amount of training hours in it. (Perhaps a couple thousand hours for running compared to many thousands of hours going uphill).
So if I run at a high heart rate, my legs are dying, and I would be sore for days. I don’t have the mechanic efficiency or strength endurance to run fast for very long without destroying my legs. In contrast, I can go uphill quite comfortably at similar heart rates without much fatigue unless it’s at a truly high intensity.
Similarly, if I did a fitness test on cross country skis, it would be laughable. My inefficiency is so bad that I probably couldn’t go very hard at all, and I would be destroyed afterward.
So with testing on a flat or inclined treadmill, I always go with an incline if I want something sport-specific. I only test on a flat treadmill out of curiosity, not to inform my training.
Does that help?