Is there such a thing as an online pcb tester? As in ‘here is my pcb design, when I connect this, I expect it to register as a circuit on these two pins’
Not that I'm aware of but the DRC systems should be able to make it pretty clear if there's track issues.
There are circuit / electrical simulators but that's a very complicated solution for keyboard designs that tend to be simplistic.
Matrix circuit designs are not much to be worried about. If you had a design in question I'm sure someone including myself would be willing to look at it.