Ignore incompatible_msrv clippy false positive in test

rust-lang/rust-clippy#12257

    warning: current MSRV (Minimum Supported Rust Version) is `1.60.0` but this item is stable since `1.64.0`
      --> tests/cxx_string.rs:16:15
       |
    16 |         g(&s).await;
       |               ^^^^^
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#incompatible_msrv
       = note: `-W clippy::incompatible-msrv` implied by `-W clippy::all`
       = help: to override `-W clippy::all` add `#[allow(clippy::incompatible_msrv)]`

tests/cxx_string.rs

@@ -1,4 +1,5 @@
 #![allow(
+    clippy::incompatible_msrv, // https://github.com/rust-lang/rust-clippy/issues/12257
     clippy::items_after_statements,
     clippy::uninlined_format_args,
     clippy::unused_async