input: Write a function to construct a naive bayes classifier using python to classify emails as spam or not spam. The function should take training data (X_train and y_train) and testing data (X_test) as input, train the classifier, and return the predicted output.
  assert naive_bayes_classifier(X_train, y_train, X_test)==y_pred
  
  where:
  - X_train: Training data features
  - y_train: Training data labels
  - X_test: Testing data features
  - y_pred: Predicted labels for the testing data
output: def naive_bayes_classifier(X_train, y_train, X_test):
      from sklearn.naive_bayes import MultinomialNB
      classifier = MultinomialNB()
      classifier.fit(X_train, y_train)
      return classifier.predict(X_test)