Try putting expected mutations inline in testdata as comments?

[sourcefrog]

Perhaps it would be congenial to have testdata like this:

fn a() -> usize {
    // EXPECT: replace + with -
    // EXPECT: replace + with *
    3 + 4

(Instead of the current large and unwieldy snapshot files like https://github.com/sourcefrog/cargo-mutants/blob/main/tests/snapshots/list__list_mutants_in_all_trees_as_text.snap)