Mass updates are like saved searches you can make updates to. You create the mass update as a saved search for customer records where email is empty. On those results, you set the email field value to
test@test.com like you said, then you click preview. If the results look correct for items to update, and the changes being made listed at the top are correct, you can submit it to start. Mass update doesn’t create new records like CSV can, but both can update records in bulk.