tests/csplit: ignore IRWXU from cspell check

tests/by-util/test_csplit.rs

@@ -1441,6 +1441,7 @@ fn test_named_pipe_input_file() {
 
 #[cfg(unix)]
 fn create_named_pipe_with_writer(path: &str, data: &str) -> std::process::Child {
+    // cSpell:ignore IRWXU
     nix::unistd::mkfifo(path, nix::sys::stat::Mode::S_IRWXU).unwrap();
     std::process::Command::new("sh")
         .arg("-c")