With crush washers widely available for less than $2 each, it hardly seems to be any bargain in terms of ease of effort to choose removing the glow plugs down in the abyss between the IP and the head with their easily dropped wire nuts, and awkward access, over removing the injectors and testing with that adapter. Just time wise alone I'd bet it takes three or four times as long to get the glow plugs out compared to getting the injectors out. And properly done a compression test should be done on an engine at operating temp so getting down to the GP's can be uncomfortable as well.

If when replacement injector return lines are installed their loops are made a couple inches longer than the minimum length required they can easily be reused by just cutting them off just above the nipple loosing only the 8 or 10 mm on each end whenever they need to be disconnected, so you can avoid replacing them in their entirety each time doing it that way as well.