getInput() doesn't return anything. Instead, if there is input, it sends an input event (e.g., an EventKeyboard) to all Objects (using Manager onEvent()). So, you could test this functionality in a variety of ways. One example:
Start up the GraphicsManager (needed since it starts up Curses)
Start up the InputManager
Create an Object (say, a Hero derived from Object)
Sleep or loop for a few seconds, giving you time to press a key
Call InputManager getInput()
If getInput() does it's job, the eventHandler() for the derived object should get a keyboard event with the key.
For automated testing, you can use ungetch() to "push" keystrokes to simulate a player pressing keys and test from there.