Very sad. I hope any future expeditions include necessary precautions to avoid reoccurrence of such a disaster.
One tests pressure (internal) vessels hydrostatically.
Using water means low potential energy if it fails,
ie, no explosion. It's tested to well above rated
operating pressure.
The same approach can be applied to submersibles.
Fill it 99.9+% with water so there's very little air volume,
but there's room for hull compression without affecting
interior pressure. Use pressure transducers to monitor
it during descent (unoccupied) to well below rated
operating depth. Then inspect it using non-destructive
methods to verify condition.