The Chernobyl test was not a test to drive the reactor to the limits, but actually a test to verify that the inertia of the main turbines is big enough to drive the coolant pumps for X amount of time in the case of grid failure.
https://www.youtube.com/watch?v=Ijst4g5KFN0
This is a presentation to students by an MIT professor that goes over exactly what happened, the sequence of events, mistakes made, and so on.