How to think of test cases

1.  Easy cases
    print_integer(5, 10, "")  ⇒ output:  "5"

2.  Edge cases  -- smallest, largest (or long enough), shortest, longest (or long enough)
    INT_MIN  -- smallest n
    INT_MAX  -- largest n
    radix=2  -- smallest radix
    radix=36 -- largest radix
    "" -------- shortest prefix
    "$€$$" ---- long enough prefix

3.  Corner cases -- values where behavior of the program changes
    n --- single digit to multiple digit is a transition
    radix -- numeric digit bases (2-10) to alphanumeric bases (11-36) is a transition.
    Test at the corner, and also directly before/after.  Ex:  n=-1, n=0, n=1