If you are using Tune_up.js(cmdline) to run your UIA tests at the end of each test function the app will reset its state.
Other than this the only things you can do to simulate closing the app would be to deactivate the app which sends it to the background for a given amount of time and then returns it to the foreground.
Additionally you could simulate locking the device, however I have never tried this specifically.
target.deactivateAppForDuration() https://developer.apple.com/library/ios/documentation/ToolsLanguages/Reference/UIATargetClassReference/UIATargetClass/UIATargetClass.html#//apple_ref/javascript/instm/UIATarget/deactivateAppForDuration