## Summary
This PR implements validation in the formatter tests to ensure that we
don't modify the AST during formatting. Black has similar logic.
In implementing this, I learned that Black actually _does_ modify the
AST, and their test infrastructure normalizes the AST to wipe away those
differences. Specifically, Black changes the indentation of docstrings,
which _does_ modify the AST; and it also inserts parentheses in `del`
statements, which changes the AST too.
Ruff also does both these things, so we _also_ implement the same
normalization using a new visitor that allows for modifying the AST.
Closes https://github.com/astral-sh/ruff/issues/8184.
## Test Plan
`cargo test`
