I2CSlave test program. I2CSlave library has some improper behavior, so this program is able to watch that. The workaround for this behavior, only for NXP device, is wrote in 'I2CSlave_mod_NXP.cpp'.
First, write this program to the microcontroller which you want to check. Second, send I2C data to the microcontroller from the other one. Anything is enough for the transmission data, but the data length is made 8, 9, 10 and 11 bytes and confirm each reply.