add a test for the TLS memory leak

tests/many-seeds/tls-leak.rs

@@ -0,0 +1,13 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/123583>.
+use std::thread;
+
+pub(crate) fn with_thread_local() {
+    thread_local! { static X: Box<u8> = Box::new(0); }
+    X.with(|_x| {})
+}
+
+fn main() {
+    let j2 = thread::spawn(with_thread_local);
+    with_thread_local();
+    j2.join().unwrap();
+}