The `y_test` argument must be supplied if forecasts are desired or if
`n_ahead` < `nrow(X_test)`. This is because in order to obtain 1-step
forecast for, say, the 10th observation in the test data set, the 9th
observation of `y_test` is required. The length of `y_test` will determine
how many forecasts to produce. In order to get true forecasts for the first
30 observations after the training set, one must (currently) produce the set
of 1-step, 2-step, 3-step, ..., 30-step ahead predictions.