input: Write a python function to simulate rolling two dice and return their sum.
  assert roll_dice_sum() in range(2, 13)
  assert isinstance(roll_dice_sum(), int)
output: import random
  
  def roll_dice_sum():
      die1 = random.randint(1, 6)
      die2 = random.randint(1, 6)
      return die1 + die2