Re: "Quality" is a structural attribute, not a bolt-on
"As the unit tests are generally written by developers"
about that. 'unit tests' aren't necessary in the majority of cases, especially when they're TRIVIAL. If the system is properly designed, i.e. broken into functional unjits correctly, you can test the overall functionality of the whole on a known dataset. This also means properly writing the thing in the FIRST place, to avoid all of the usual problems (memory leaks, buffer overruns, mishandled erroneous data, use-after-free pointers, etc.).
In other words, "unit test" for every trivial freaking thing is what JUNIOR coders do, because they can't see past their own noses and look at the BIG picture.
(as for making up some kind of test data to verify an algorithm, that's not the same... you use that to WRITE the algorithm, and once done, you NEVER! HAVE! TO! TEST! IT! AGAIN!!!)