These firms state they provide proof testing. The usual load would be 1 1/2X the pressure of the load you intend to use.
I have no personal experience so please let us know what you find out.
H.P. White is no longer is business and quite some time ago the cost was $400

Entropy Engineering Corporation
http://www.entropyec.com/


Chesapeake Testing
https://www.elementdefense.com/services


Dayton T. Brown
https://www.dtb.com/