input: Write a function to convert a given string to a list of integers.
  
  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('-5,-3,-1,0,2,4') == [-5, -3, -1, 0, 2, 4]
  assert to_int_list('0') == [0]
  assert to_int_list('999') == [999]
output: def to_int_list(s):
      return [int(x) for x in s.split(',')]