OpenSprinkler firmware does allow you to run a program on the controller by using buttons. Specifically, you can press and hold pushbutton B3 for a few seconds until the LCD screen displays ‘Run Program’. Then click B3 to cycle through a test program and all existing programs. Once you’ve found the program, press and hold B3 again and the program will start running.
If you want to set an ad-hoc program that you don’t want to run normally, but just for testing purpose. You can set the program to be ‘disabled’. That way, the program won’t run normally, but you can still start it manually using buttons (or on the UI/app by using ‘Run this program now’ button)