pthread-threadname: ensure we can see the name set via the Rust API

src/tools/miri/tests/pass-dep/libc/pthread-threadname.rs

@@ -65,6 +65,22 @@ fn main() {
         }
     }
 
+    // Set name via Rust API, get it via pthreads.
+    let long_name2 = long_name.clone();
+    thread::Builder::new()
+        .name(long_name.clone())
+        .spawn(move || {
+            let mut buf = vec![0u8; long_name2.len() + 1];
+            assert_eq!(get_thread_name(&mut buf), 0);
+            let cstr = CStr::from_bytes_until_nul(&buf).unwrap();
+            let truncated_name = &long_name2[..long_name2.len().min(MAX_THREAD_NAME_LEN - 1)];
+            assert_eq!(cstr.to_bytes(), truncated_name.as_bytes());
+        })
+        .unwrap()
+        .join()
+        .unwrap();
+
+    // Set name via pthread and get it again (short name).
     thread::Builder::new()
         .spawn(move || {
             // Set short thread name.
@@ -130,6 +146,7 @@ fn main() {
         .join()
         .unwrap();
 
+    // Set name via pthread and get it again (long name).
     thread::Builder::new()
         .spawn(move || {
             // Set full thread name.