input: Write a python function to get the corpus of the most active users from a dataset.
  assert len(get_corpus_of_most_active_users(n_users=3)[0]) == 3
  assert isinstance(get_corpus_of_most_active_users(n_users=2), tuple)
  assert isinstance(get_corpus_of_most_active_users(n_users=2)[0], list)
  assert isinstance(get_corpus_of_most_active_users(n_users=2)[1], list)
output: def get_corpus_of_most_active_users(n_users):
      from collections import Counter
      import random
  
      # Sample data: list of tuples (user_id, post)
      data = [
          (1, "This is the first post by user 1"),
          (2, "This is the first post by user 2"),
          (1, "This is the second post by user 1"),
          (3, "This is the first post by user 3"),
          (2, "This is the second post by user 2"),
          (3, "This is the second post by user 3"),
          (2, "This is the third post by user 2"),
          (1, "This is the third post by user 1"),
          (4, "This is the first post by user 4"),
          (4, "This is the second post by user 4"),
          (5, "This is the first post by user 5")
      ]
  
      # Count the number of posts by each user
      user_post_count = Counter([user_id for user_id, _ in data])
      
      # Get the most active users
      most_active_users = [user for user, count in user_post_count.most_common(n_users)]
      
      # Get the corpus of posts from the most active users
      corpus = [post for user_id, post in data if user_id in most_active_users]
      
      # Split corpus into list of lists, each sublist contains posts of one user
      user_corpus = {user: [] for user in most_active_users}
      for user_id, post in data:
          if user_id in most_active_users:
              user_corpus[user_id].append(post)
      
      # Sort users by their activity level
      sorted_users = sorted(user_corpus.keys(), key=lambda x: user_post_count[x], reverse=True)
      
      # Get the corpus in the form of list of lists
      corpus_list = [user_corpus[user] for user in sorted_users]
      
      return corpus_list, most_active_users