i think your making this up
what happens when the plug fails short current ? - it'll still show battery voltage on your voltmeter and would light up your bulb, which by your methods would indicate a working plug
your test will only indocate an open circuit - as plugs are of very low resistance anyway the only proper way to test them is to remove them and visually watch to see how long it takes them to glow red when you have the glow plug wiring attached and not when you've rigged them up to a PP3 using bell wire
a good 200/300 wont need the assistance of plugs unless it is very sub zero
