Suggest you call a few valve manufacturers and see what they use if you are referring to ball, gate, plug, etc. type valves. Would not be surprised if they build it themselves as the test instruments would be similar to that used to hydro test piping.
As a point of interest, check the test times listed in either API 6D or ASME B16.34. Not very long.
If you are talking about testing PSV set points then most people try to test in place. For small valves install external valves. For large pilot operated valves buy a test kit for the valve.
For testing the flow capacity of PSVs I do not know anyone who performs this type of testing after it is in service.