Use a backslash escaping heuristic to keep non-path backslashes around

[oli-obk]

rust-lang/rust-clippy#10426 (comment)

Could we adapt the path heuristic from rust's compiletest here? I think it would need support in ui_test as it uses a non string Replacer

It would be nice to have non path backslashes stay as backslashes as that frequently confuses people writing tests