Apple simplified iOS UI testing in Xcode 7.
With new Xcode 7 projects, all you have to do to get the complete configuration for iOS UI testing is to check "Include UI tests" when creating a project.
What are the steps to get the same in Xcode 7 for projects that were created before Xcode 7 existed?
After testing several older projects:
Adding a new UI test target is sufficient as suggested by a-live in a comment to the question:
File > New > Target > Test > Cocoa Touch UI Testing Bundle
With Xcode 7 beta1, all of my projects habe been unstable after adding this target with different symptoms:
Most problems were fixed after rebuilding the project, exiting XCode, restarting XCode with the project, rebuilding again. All were fixed after repeating this.