input: Write a python function to train and test a Gaussian Naive Bayes model using sklearn.
  assert train_and_test_gaussian_nb(X_train, y_train, X_test) == y_pred
output: def train_and_test_gaussian_nb(X_train, y_train, X_test):
      from sklearn.naive_bayes import GaussianNB
      model = GaussianNB()
      model.fit(X_train, y_train)
      y_pred = model.predict(X_test)
      return y_pred