In the bowl of a food processor, combine garlic scapes, Parmesan, pistachios, salt, and pepper, and pulse to coarsely chop. While machine is running, gradually add oil until pesto is pureed but still a bit chunky. Set aside. Bring a medium pot of water to a boil over high heat. Add fava beans and cook for 3 minutes. Drain and rinse beans with cold water. Remove the tough outer skin of each by pinching the outer skin to pop out the darker-green bean inside. Discard skins and place beans in a large bowl. Add pasta and feta, and then toss with pesto to coat. Serve at room temperature or after chilling.