Where does fuel from the boost pumps go before engine start?
I can’t say for sure about the jet engine mentioned. If I think of it like a piston engine, the fuel pump pressurizes the fuel lines. That in no way means that the fuel is entering the combustion part of the engine (cylinders in my case). It just means that the fuel lines leading to the fuel distribution point (carburetor or EFI) are being fed fuel at a certain pressure. A primer is a different story. It is feeding fuel directly into the combustion chamber by using pressure. 

As mentioned in another post, more pressure and fuel is supplied by the fuel pump than is needed by the engine. In the case of a Cessna 172SP, the excess is routed to a reserve fuel reservoir. Here is a schematic of the fuel system from the POH.