How do I take screenshots of my UI with Xcode 7 during UI Testing?

If you want to generate screenshots, you can also use snapshot, which describes how to trigger screenshots in UI tests: https://github.com/fastlane/fastlane/tree/master/snapshot#how-does-it-work

It basically rotates the device to .Unknown (Source), which triggers a snapshot without actually modifying your app's state.

Comparing the output with the generated plist file enables you to even properly name the screenshot

UI Testing in Xcode automatically takes screenshots of your app after every step.

Simply go to one of the tests that has already ran (Report Navigator > choose a Test), then start expanding your tests. When you hover your mouse over the steps, you will see eye icons near each step that has a screenshot.

Here's an example... in this case, notice the eye icon at the end of the gray row. If I were to tap on it, I would see a screenshot of the app right when the button in my app was tapped (since the step is Tap the "Button" Button).