Used as a subroutine in InvariantTargetPrediction to test whether out-of-sample prediction performance is better when using X and E as predictors for Y, compared to using X only.
predictedOnlyX: Predictions for Y based on predictors in X only.
predictedXE: Predictions for Y based on predictors in X and E.
verbose: Set to TRUE if output should be printed.
...: The dimensions of X (df) and E (dimE) need to be passed via the ... argument to allow for coherent interface of fTestTargetY and wilcoxTestTargetY.