Just in case anyone is interested, here is a picture of the simple test board I made for the REV B AGC kits. It uses an ATtiny85 to generate a ~700Hz square wave which is reduced to the correct level using a potential divider. The ATtiny then samples the muting line to determine that tone application causes muting. LEDs provide a simple test result to the user.
It’s worth noting, every QCX AGC is tested before shipping to ensure the quality is up to standard.
So far I haven’t found any defective units