How to access back bar button item in universal way under UITests in Xcode?
Generally the back button tends to be the first button element in the navigation bar
app.navigationBars.buttons.element(boundBy: 0).tap()
Based on Aaron Sofaers comment, you can also set the accessibilityIdentifier directly in Interface Builder.
Here's how you can do it in Swift 3:
app.navigationBars.buttons.element(boundBy: 0).tap()