Yes, it is expected and polite to credit your testers in the game, unless a tester asks you to leave out their name. In a parser game, it is common to acknowledge testers in response to a command like ABOUT or CREDITS, or as a menu option if you have a menu.
Players--especially during a competition--may check to see if your game lists any testers before they start playing. Some players, if they don't see any testing credits, may decide not to play the game at all, out of concern that the game may be untested and buggy.