thanks for you efforts! This is great and really useful work. IMO it is the right direction, even though I always hoped to find a way around implementing our own I2C slave device. That's why I wanted to take this opportunity to ask around for other experiences with I2C (slave) test devices. Anyone, anything?
I have done it the other way around from what Hauke's doing it: I used a master as the tester device, to develop the driver for a slave. The driver was already implemented on the master, which was an Arduino, so I just wrote a bunch of test cases (correct and incorrect message formats) and then ran them on a loop on the Master with serial output onto my computer. (No test logic on the computer, just text output.)
So I guess you could maybe use an Arduino as the slave and write some basic tests using their "Wire" (basically, I2C) library?
I'm also doing something similar to this with uart testing. Will update re thoughts on that once I've had more of a play around.