A decent check for auto bed-leveling on the CR10

I recently installed a BLTouch sensor on my bed.

More info here.

I needed a way to evaluate the z-probe offset after leveling, but I couldn't find a good test for auto bed leveling, so I made my own. It's especially important on the CR-10 since the bed is so large. This is based on the Prusa first layer calibration routine with a zig-zag across the whole bed, then a small square to test perimeter adhesion.