I've got an ongoing relationship with a long term business partner as a consultant where his role is project manager (task manager + direction), and my role is a contracted developer. He has a tendency to micromanage my time with his tasks and oversight, but also has a strong sense of perfection.
Recently with every single programming task undertaken he is asking me to confirm that I have "100% confidence that this fix will not break any existing features or cause any adverse effects on user experience". If I can't affirm that, he assumes I haven't tested it well enough or should go check it again. And yes, he actually asks this every single bug fix, it isn't just implied.
As a developer, I do test my work on multiple unit cases, but can't say that it is possible to fully regression test the entire product for each 2-hour task I accomplish. There is no QA team either. The product has lots of intermingled parts throughout (not just self-contained pages), some 40,000 lines of code written over 4 years, and sometimes unexpected things happen that we weren't even aware of. I sense he sees this as poor testing.
How should I respond to his question in this case, without seeming incompetent? Honestly I never have 100% confidence sitewide, but I do have confidence in my testing methods. And, as a developer I also know that it isn't uncommon for unexpected bugs to emerge later from these core changes.
I'm not necessarily looking for a solution to make this 100%, as our group doesn't have the time or resources to implement a full QA process or get into setting up automated solutions. I'm looking for how to interact with the manager around existing work, especially when he isn't entirely a technical person himself. He isn't a programmer.