Fix SAT test

The SAT collision test in core was expecting the wrong overlap.  It uses `Crafty.circle` to generate two octagonal polygons; their minimal overlap is actually `-15 cos[pi/8] ~= -13.86`.  The previous incorrect overlap wasn't completely arbitrary: it was `-20 cos[pi/8]`.

For reference, two perfect circles would have an overlap of -15.  But this is testing that SAT works, not how well we approximate circles!