First off, they didn't simply put the battery in a steel box - the entire system was redesigned. The battery was redesigned with increased isolation between the cells to prevent a cell failure from propagating to adjacent cells, the charging system was redesigned to ensure there is no overcharging (which can damage battery cells), and then - just to be sure - they put the battery in a steel box.
And yes, they did a test where they intentionally failed a battery - creating a complete battery meltdown - inside the steel case and confirmed it performed as advertised.
BTW, Airbus had every intention of using Li Ion batteries in the A350 before Boeing ran into problems - at which point they decided that there was too much risk and went with an older battery technology.