[test] add Unix-only tests for with_suppressed_stderr ensuring stderr redirection and restoration, including panic handling

tests/with_suppressed_stderr.rs

@@ -0,0 +1,58 @@
+// Unix-only tests for with_suppressed_stderr restoring file descriptors
+// Skip on Windows and non-Unix targets.
+
+#![cfg(unix)]
+
+use std::panic::{catch_unwind, AssertUnwindSafe};
+
+fn stat_of_fd(fd: i32) -> (u64, u64) {
+    unsafe {
+        let mut st: libc::stat = std::mem::zeroed();
+        let r = libc::fstat(fd, &mut st as *mut libc::stat);
+        assert_eq!(r, 0, "fstat failed on fd {fd}");
+        (st.st_dev as u64, st.st_ino as u64)
+    }
+}
+
+fn stat_of_path(path: &str) -> (u64, u64) {
+    use std::ffi::CString;
+    unsafe {
+        let c = CString::new(path).unwrap();
+        let fd = libc::open(c.as_ptr(), libc::O_RDONLY);
+        assert!(fd >= 0, "failed to open {path}");
+        let s = stat_of_fd(fd);
+        let _ = libc::close(fd);
+        s
+    }
+}
+
+#[test]
+fn stderr_is_redirected_and_restored() {
+    let before = stat_of_fd(2);
+    let devnull = stat_of_path("/dev/null");
+
+    // During the call, fd 2 should be /dev/null; after, restored to before
+    polyscribe::with_suppressed_stderr(|| {
+        let inside = stat_of_fd(2);
+        assert_eq!(inside, devnull, "stderr should point to /dev/null during suppression");
+        // This write should be suppressed
+        eprintln!("this should be suppressed");
+    });
+
+    let after = stat_of_fd(2);
+    assert_eq!(after, before, "stderr should be restored after suppression");
+}
+
+#[test]
+fn stderr_is_restored_even_if_closure_panics() {
+    let before = stat_of_fd(2);
+    let res = catch_unwind(AssertUnwindSafe(|| {
+        polyscribe::with_suppressed_stderr(|| {
+            // Trigger a deliberate panic inside the closure
+            panic!("boom inside with_suppressed_stderr");
+        });
+    }));
+    assert!(res.is_err(), "expected panic to propagate");
+    let after = stat_of_fd(2);
+    assert_eq!(after, before, "stderr should be restored after panic");
+}