I believe that the test is generally done to check flow capability of the system without the pump(s) operating. I might be wrong as I’ve never built an airplane that required a fuel pump, and fuel injection adds another factor. If the engine won’t run at all without a pump, then there would seem to be little reason to test without. Checking with the pumps running is a good idea anyway of course, but it’s good to know what the flow rate is if a pump fails.