input: Write a python function to print an asterisk triangle of a given size.
  assert asterisk_triangle(3)=="* \n* * \n* * * \n"
  assert asterisk_triangle(4)=="* \n* * \n* * * \n* * * * \n"
  assert asterisk_triangle(2)=="* \n* * \n"
output: def asterisk_triangle(size):
      result = ""
      for i in range(1, size + 1):
          result += "* " * i + "\n"
      return result