C128 Factory Test is a bunch of testing and diagnostic programs for the Commodore 128, Commodore 128D and 1571 Floppy drive.  As you can see from the screenshots it also covers the 1700 and 1750 RAM expanders.

GMT (GGLABS Memory Test) for the Commodore 128 and geoRAM is a newly developed memory test for vintage computers optimized for speed and coverage. The code is based on the test algorithms described in "Testing Semiconductor Memories: Theory and Practice" by A.J. van de Goor.

GMT (GGLABS Memory Test) for the Commodore 128 and geoRAM is a newly developed memory test for vintage computers optimized for speed and coverage. The code is based on the test algorithms described in "Testing Semiconductor Memories: Theory and Practice" by A.J. van de Goor.

Input Device Tester 128 by Pastbytes (Released Dec. 22nd, 2022) - Here is a really slick diagnostic tool for C64 Joysticks (with up to 3 buttons), Analog joysticks, Commodore 1351 mouse (in port 1), Paddles, Keyboard and the Retropad.  

Here's a link to the release announcement over at Lemon64.

System Check 128 by R. Croswell (12/20/1992) is a diagnostic program for the Commodore 128.  Neat thing about this program is it will also show you all the hidden easter egg text in ROM.

The VIC-IIe D030 Test v2.0 is a diagnostic test specifically designed for the VIC-IIe video chip used in the Commodore 128 computer. The VIC-IIe (Video Interface Chip) is responsible for handling graphics and video output in the Commodore 128's 40-column display mode, which is primarily used in the Commodore 64 compatibility mode.