Here is the programming board that is used to program SensorDots firmware and test that pins are working. Each board segment's supply rail is separately controlled by a MOSFET connected to a TI TCA6424A 24-bit GPIO expander.
With this, each board is programmed and tested in sequence, which removes the need to gang up multiple programmers (as shown here for example - https://www.sparkfun.com/tutorials/233).
Each pin under test is connected on the same bus to simplify wiring and remove the need for more bus expanders. The downside to this is that it cannot detect which board is shorting if there is a short on a pin, but this is pretty rare (and can be tested for by hand).
Testing is performed by the onboard Teensy (to the right) and there is a ICSP header to connect to a programmer, as well as test routines in the firmware of the boards.