That single wire goes to the ecm...you don't need to test the wire.
The switch supplies a ground to that single wire when the throttle closes it.
The design principal of this "single wire type switch" is that it must be securely mounted on metal, so that technicaly it's "exterior case" is grounded. For the purposes of this explanation, that grounded metal source is in fact the manifold.
That way when it operates (correctly) it's able to pass that ground to the single wire, which in turn passes it onto the ecm.