useTestingSet [<example-set>]
With no argument, this returns a list of all example sets compatible with the active network. Given the name of an example set, this will make that set the testing set for the active network. With the null string in place of an example set, this will set the network to have no testing set.
To make "lots-o-data" the testing set:
lens> useTestingSet lots-o-data
To give the network no testing set:
lens> useTestingSet {}