I would be somewhat concerned with a tester that used the injector openings.
The tester would need to exactly displace the volume of the injector in order to get accurate readings. If the tester allows any air space, where there would normally be the injector, the results will be artificially low.

This also applies to the glow plugs. The glow plug takes up space. If the tester does not take up the same space, the results will not be accurate.

But, the glow plug is much smaller than the injector.