Mutation test brainstorming #24949

Hi, everyone!

I've been researching about mutation tests for Bitcoin Core since a while. I'm opening this PR to concentrate the discussions about it here since I couldn't find any similar issue. Some questions:

• Have you ever used any mutation lib/app for C++? Which are? Pro and cons?
• How could we use it here?
• What approaches are important to have? What do we want to mutate? Operators? Strategies?

I met universalmutator by #22690 and I have been using it and just noticed some pros and cons. I just added it on my Core fork so you can test it as well. See: https://github.com/brunoerg/bitcoin/blob/2022-04-universal-mutation/test/mutation/mutation.py

Pros: Use regex for mutation, scriptable, can be used in CI (although I think we shouldn't btw) and the module to analyze the 'mutators' is very good. We can set different "functional/unit/whatever" script for each file we've mutated (some libs/apps only allow you to set one script so it will be used for all 'mutators', this is bad because we don't want to execute test_runner ou make check for each one). See [here](" https://github.com/brunoerg/bitcoin/blob/2022-04-universal-mutation/test/mutation/mutation.py") I have a dictionary where I put the file to be mutated and the functional test to test it.

Cons: It's not focused 100% on C++ - it's a generalist tool, I had to open a PR to include some C++ rules.

I've also been testing mutate_cpp and I'll give my feedback here soon.