You don’t have to solder anything on a PCB before testing it. Just plug it in and bridge the sockets with tweezers.

Edit: I guess some of the barebones builds that are seen in the community will often need soldering prior to testing (diodes, etc.), but these are not common and the majority of keyboard PCBs can be tested without the need to solder.

Just plug it in and use a pair of tweezers to link the sockets. When they’re good to go, the press will register on whatever software you’re using.

Start with enough for a TKL, end with enough for a 40.