cli/test: Skip `rad_diff` on macOS — heartwood

macOS’s sed requires an argument for -i, which we don’t provide in the example. Providing it makes the test fail on Linux. Since this command is deprecated anyway, we just skip macOS.

 #[test]
 fn rad_diff() {
     if std::env::consts::OS == "macos" {
         // macOS's `sed` requires an argument for `-i`, which we don't provide
         // in the example. Providing it makes the test fail on Linux.
         // Since this command is deprecated anyway, we just skip macOS.
         return;
     }
     let tmp = tempfile::tempdir().unwrap();
     fixtures::repository(&tmp);