input: Write a python function to generate a random 3x3 tic-tac-toe board.
  assert len(generate_board()) == 3
  assert all(len(row) == 3 for row in generate_board())
  assert all(cell in ['X', 'O'] for row in generate_board() for cell in row)
output: import random
  
  def generate_board():
      return [[random.choice(['X', 'O']) for _ in range(3)] for _ in range(3)]