input: Write a function to convert a given string to a list of integers. The function should split the string by commas and convert each substring to an integer.
  assert to_int_list("1,2,3,4,5")==[1, 2, 3, 4, 5]
  assert to_int_list("10,-20,30,40,50")==[10, -20, 30, 40, 50]
  assert to_int_list("100,200,300")==[100, 200, 300]
  assert to_int_list("-1,-2,-3,-4,-5")==[-1, -2, -3, -4, -5]
  assert to_int_list("5,4,3,2,1")==[5, 4, 3, 2, 1]
output: def to_int_list(s):
      return [int(x) for x in s.split(',')]