The only document I can think of is something I am unfortunately not familiar with, but I will risk commenting anyway.  The Boiler and Pressure Vessel code has requirements for qualification of brazers and braze procedures, but I doubt it tells you which specific tests must be done on a special joint.  For mechanical integrity, a pull test might be sufficient to show the TC sheath will fail before the brazed joint, but since the sheath is being pushed out by the pressure, a pull test may not be relevant, so a pressurized proof test would be good. You probably want to pressurize to some overpressure, such as 1.5 times the max operating pressure.  The Code may specify what is needed.  For leaks, testing with air may be sufficient.  Helium may be overkill unless leakage is critical, such as with fuels or refrigerants.  Will the completed vessel undergo a proof pressure and leak test? If so, the brazed joint integrity may be sufficiently proven as part of the vessel testing.