PETE'S TEST CARTRIDGE
If you haven't realized it from looking at the screenshots already, Pete's Test Cartridge is not a game. Similar to the 5200 Diagnostic Cartridge, this system utility essentially tests for defects in key areas. Using a standard 5200 controller to select various options, you can test for brightness (or dimness), color balance and contrast and joystick calibration, among other things.
Whereas the standard Diagnostic Cartridge which was sold to authorized Atari service centers and even used internally by Atari itself, it's not known whether Pete's Test Cartridge was used for testing purposes in any capacity, given that only one of these carts has ever been found.