input: Write a python function to convert a comma-separated string into a list of integers.
  assert str_to_list("12,34,55,67") == [12, 34, 55, 67]
  assert str_to_list("1,2,3,4,5") == [1, 2, 3, 4, 5]
  assert str_to_list("10,20,30") == [10, 20, 30]
output: def str_to_list(s):
      return [int(x) for x in s.split(',')]