The “Hyper Suprime-cam” instrument came online in 2012, so it’s not new, and was presumably calibrated against well-understood targets; so its light-detection capabilities have been well tested. The physics of gravitational lensing is also well understood and verified, so they know what light they’re looking for.
If you can test each part of a system independently, and the parts connect in a reasonably trivial fashion, it’s about as good as testing the whole.
Depends. In functional programming, if all components of a pipeline are tested independently, you can all but guarantee that the pipeline as a whole will work as intended.
That said, software testing isn't really an apt analogy now that I give it some thought. Testing real world equipment has to contend with a lot more edge cases and environmental factors.
If you can test each part of a system independently, and the parts connect in a reasonably trivial fashion, it’s about as good as testing the whole.