Using the Samsung stop watch app. I started it running, selected the back button, selected the home button, swiped right to the calendar widget, selected to open the calendar x viewed the next events, selected the home button, selected the home button again to open the app screen, selected the stopwatch app, the stopwatch app opened and the tone had kept advancing.
I use the Samsung timer nearly every day. It is set as my double tap home app. I can navigate away from the timer app and do all sorts of things and the timer will continue to run in the background and sound the alarm upon completion.