input: Write a python function to simulate a simple tic-tac-toe game where two players randomly place 'O' and 'X' on a 3x3 board.
  assert len(simulate_tic_tac_toe()) == 9
  assert simulate_tic_tac_toe().count("O") == 2
  assert simulate_tic_tac_toe().count("X") == 1
output: import random
  
  def simulate_tic_tac_toe():
      board = [' ' for _ in range(9)]
      moves = ['O', 'X']
      random.shuffle(moves)
      
      for move in moves:
          while True:
              position = random.randint(0, 8)
              if board[position] == ' ':
                  board[position] = move
                  break
      
      return ''.join(board)