might possibly get you through the test. Breathe steadily and much more deeply than normal, for several minutes. This saturates your blood with more oxygen
than necessary, so the sensors which tell the brain how much blood the the organs need, are fooled, and the pulse-rate drops. But the effect is only temporary of course.
Breathing oxygen beforehand also accomplishes the same thing. You can buy "athletic" shot-containers with a can and a mask, legally over the counter. But again, it's a temporary manoeuvre, as th excess oxygen is used up.
Thirdly, you might well pass the step-test just by breathing very deeply -and perhaps a bit quicker, while you do the stepping. This has the same effect as the first suggestion, but lasts all the time you are being tested.
However, I'd be a bit dismayed at failing the test if I were you? There must be a reason, and you'd be well advised to find out why, for your own peace of mind. Are you obese? Overweight? unfit? Heart/lung troubles? Why not find out?