Hi Jay. Typically an unlabelled Test set is provided by the machine leaning practitioner, or by the problem statement, or by the competition. Please see here for options for adding a Test set to the DataBunch:
If you want to split out your own Test set, IMHO the easiest way is to learn how to use DataFrames (Pandas). With DataFrames you can load in the csv, take a sample, write a new CSV, move files, etc. Then fastai will split the remaining set into Training and Validation.