At every boot, BHI160 will initialized each part and will only succesfully boot if each slave answers, so it is guaranteed that communication work. But that is not a true self-test. Luckily the self-test code for each slave is pre-programmed in the Sensor Hub.
The init function typically boots up the hub directly, which is why you don't get self-test results. to trigger it, this piece of code is the important bit: