I have never tested for a spark in the way you describe in the last part of your post ie “Plug cap to plug terminal no light.” All I do is test plug lead to a good earth on engine, then connect plug lead to plug and hold or clamp plug to a good earth on the engine.
You will not get a spark from plug lead (cap) to plug terminal when it is screwed into the engine as there is no earth path, the centre electrode of the plug is insulated from the engine and can only spark across the plug gap. What you are doing is introducing a second gap in series with the plug gap, the attached sketch illustrates this.
