Is there some formal way(s) of quantifying potential flaws, or risk, and ensuring there’s sufficient spread of tests to cover them? Perhaps using some kind of complexity measure? Or a risk assessment of some kind?
Experience tells me I need to be extra careful around certain things - user input, code generation, anything with a publicly exposed surface, third-party libraries/services, financial data, personal information (especially of minors), batch data manipulation/migration, and so on.
But is there any accepted means of formally measuring a system and ensuring that some level of test quality exists?
Different applications require different tests, so no measure is going to please everyone. If you’re making embedded devices for an airplane, the buyer might ask you to provide a formal proof that the program works. In contrast, web apps tend to simply use end users as testers, since it’s cheaper.