There are a few cheats on this game all of which is found online, but I am only listing what seems like the toughest at first glance and some cool hidden stuff for the game.
On title screen an easy way to enter test mode is to hold a and b on controller 2, adjust if using PlayStation controller which should be X and O, then with those held down go into settings, choose restart with controller 1 and before you confirm to restart hold down on controller 2. With the two buttons pressed and while holding down on controller 2 on controller 1 confirm to restart.
There is indeed a secret extended ending to this game. To get it you must collect two orbs with one found on level 3 and the last one on final level 7.
How to collect orbs for secret ending.
On level 3 after beating fly you notice two containers, one of which on the left is open. After beating go to far left one and press up. You will then be teleported to a secret area. Reach the far end for a Japanese dance cutscene and you will get the orb at the end.
Final orb on level seven can be found by dropping down into first dropoff and landing behind guy. If you mess up rewind and try again. Follow, but do not attack. Enter room at far end will take you to Egypt. Do the same as above and reach far end of secret level to collect.
On the dancing Japanese lady scene you can fart several times, which if caught by her sends her into rage. Mash A and B button or X and O several times to make this happen.