In my experience you can get surprisingly far with automated fuzz testing and runtime checks. Doing it well is a bit of an art form - it’s quite easy to accidentally write fuzz tests which almost never exercise some code paths. Or end up with fuzzers that spend 99.9% of your cpu checking invariants and things like that. But with a good fuzzer, the ratio of bugs found per line of testing code is insanely high. And they’re much easier to maintain than a big bank of unit tests.
Highly recommended if you want to get more bang per line of testing code.
In fact, in my current project, I add every single path found by AFL++ to my test suite. It tests all kinds of terrible things, and then those paths are tested forever.
I even run all the paths AFL++ finds through the sanitizers and Valgrind.
Highly recommended if you want to get more bang per line of testing code.