Which one should be used in the following statement?
It's pointless searching for new path, unless all the known routes has been tested.
It's pointless searching for new path, unless all the known routes have been tested.
You need to understand difference b/t has been and have beenfor the sake of future usage.
You use has when there is a singualr subject = Ex: She has a car / She has been on accident.
You use have when there is plural subject = Ex: They have a car / They have been playing all night.
But in your case, you have to use have because "all the known routes" is a subject. ALL indicates plural form of subject therefore you use have instead of has