stepify does not modify its arguments because they are numbers, instead it provides the result as a return value.
However you might not want to apply this to test directly because that would cause the tiny difference added by delta to be lost.
If you want to see the stepified result, you can write:
print(stepify(test, 4))
Which should give you a result like a bunch of 0 for 4 seconds, then 1 until 8 seconds, then 2 etc.