🧪 Testing: Rename "mutation" tests to "integration"

[JoshuaKGoldberg]

Overview

I originally named "mutation" tests in this library and auto-mutate/automutate-tests as such because they were testing "mutators". The naming all made sense.

But, "mutation testing" is separately a thing! https://stryker-mutator.io. IMO it's confusing to use the phrase in a way different from the industry norm.

Proposal: let's switch the name for the concept in this repo to... say, integration tests? That's basically what they are, I think. WDYT @rubiesonthesky?