The 2x wood would act as a buffer and catch shrapnel, without providing structural resistance. CIP concrete would probably be a better solution, since you could more closely space reinforcement, providing better blast integrity of the wall. That said, the energy you would be absorbing and resisting should be mainly from the impact of small shrapnel, if the test is properly designed. By properly designed, I mean that the pressurization has to be through a proper hydrotest unit which would necessarily limit the available energy in the event of a failure. The likely failure mode in this case is valve rupture without generating flying debris.
If they want to use a frack pump to supply the pressure, they aren't testing it for safety, they are proofing it. An entirely different bunker design, and one that would require knowing the size and mass of the unit, pressurized volume and elasticity of the system, the known failure modes, etc. Otherwise, it's all a guess.