input: Write a function to convert a given string to a list of integers by splitting the string by commas and converting each element to an integer.
  assert to_int_list("1,2,3,4,5")==[1, 2, 3, 4, 5]
  assert to_int_list("10,20,30")==[10, 20, 30]
  assert to_int_list("5,10,15,20,25,30")==[5, 10, 15, 20, 25, 30]
  assert to_int_list("1,3,5,7,9")==[1, 3, 5, 7, 9]
  assert to_int_list("0,0,0,0,0")==[0, 0, 0, 0, 0]
output: return [int(x) for x in s.split(',')]