input: Write a python function to generate a random string consisting of special characters.
  assert len(generate_random_string()) == 8
  assert all(char in '@#$%^&*' for char in generate_random_string())
  assert generate_random_string() != generate_random_string()
output: import random
  
  def generate_random_string():
      special_characters = '@#$%^&*'
      return ''.join(random.choice(special_characters) for _ in range(8))