input: Write a python function to load a dataset from a CSV file and split it into training and test sets.
  assert isinstance(load_and_split_dataset("reviews.csv"), tuple)
  assert len(load_and_split_dataset("reviews.csv")) == 2
output: def load_and_split_dataset(filename):
      import pandas as pd
      from sklearn.model_selection import train_test_split
      
      # Load the dataset
      data = pd.read_csv(filename)
      
      # Split the dataset into training and test sets
      train_set, test_set = train_test_split(data, test_size=0.2, random_state=42)
      
      return train_set, test_set