A common resume writing advice is to support achievements with numbers. Currently, I have this line on my resume:
Migrated whole test suite comprised of 100+ GUI tests and 700+ system tests to the next major release of the operating system with minimal modifications.
I've already sidestepped the issue that they were still testing on an old operating system whose extended life cycle support ended a year ago and which no customer ran our application on. It wasn't a difficult task, but neither was it trivial.
Most problems I solved were due to the nature of GUI/system tests and how badly they were written in the first place, and the non-straightforwardness of Linux font configuration, so leaving the type of tests out would undermine my accomplishment, while keeping it would, in my opinion, reflect negatively on the company, because that's an unhealthy ratio of tests to have (there were neither unit nor integration tests). The large number of tests was intended to put things into perspective and show that I couldn't possibly have brute forced my way through the problem.
Another common resume writing advice is to be specific about the achievements. The thing is I'd worked on adapting feature F from product A to product B, but feature F on product B ran extremely slow, and made even much slower (original time complexity squared) after a colleague took over from where I left off, and remained as slow after he passed the work on to a more senior developer. So I'm afraid that, on a slim chance, feature F on product B had gained a bad reputation in the industry, and I would be viewed negatively by association, unjustly. I don't want to leave it off either as that represented an important bulk of my rather short employment period.
How could I put them more positively and appealingly to potential employers?