This question has been asked and answered already… See here how to create a second DataBunch when you want to validate you model on another set.
As for the F1 score, it is available on non-multi-class data if you use FBeta(beta=1) (another question that has been asked a lot on the forum).
Why does it make sense to have no labels for your test set? Well, when you are in a Kaggle competition, your test set doesn’t have labels and you need to provide predictions. The fastai test set is there for that. If all you want to do is validate, use the validation set, with another DataBunch if you have another validation set.