input: Write a python function to print a square pattern of asterisks of a given size.
  assert print_square(4)=="****\n****\n****\n****\n"
  assert print_square(2)=="**\n**\n"
  assert print_square(3)=="***\n***\n***\n"
output: def print_square(size):
      square = ""
      for _ in range(size):
          square += "*" * size + "\n"
      return square