MDL-41756 (1) quiz statistics : use new method increment_progress

to cut down on clutter before adding new break down by variant stuff
MDL-41756 (2) quiz statistics : break down question stats by variant
MDL-41756 (3) quiz statistics : test variant stats
MDL-41757 quiz statistics : limit subq and variant stat rows

and print min, median and max.
MDL-43338 quiz statistics : refactoring question statistics code

to improve readability and maintainability.