Suppress ptr_arg clippy lint in test suite
Suppress ptr_arg clippy lint in test suite

    warning: writing `&String` instead of `&str` involves a new object where a slice will do
       --> tests/test.rs:207:24
        |
    207 |     fn callback_ref(s: &String) {
        |                        ^^^^^^^ help: change this to: `&str`
        |
        = note: `#[warn(clippy::ptr_arg)]` on by default
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg

dtolnay/cxx,dtolnay/cxx,dtolnay/cxx,dtolnay/cxx