input: Write a python function to filter posts by user_id.
  assert filter_posts_by_user_id(posts, 123) == [{'user_id': 123, 'title': 'My post 1', 'content': 'Lorem ipsum dolor sit amet...'}, {'user_id': 123, 'title': 'My post 2', 'content': 'Lorem ipsum dolor sit amet...'}, {'user_id': 123, 'title': 'My post 3', 'content': 'Lorem ipsum dolor sit amet...'}]
  assert filter_posts_by_user_id(posts, 999) == []
output: def filter_posts_by_user_id(posts, user_id):
      return [post for post in posts if post['user_id'] == user_id]