Add barrier tests

tests/run-pass/concurrency/barrier.rs

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+// ignore-windows: Concurrency on Windows is not supported yet.
+
+use std::sync::{Arc, Barrier};
+use std::thread;
+
+// Check if Rust barriers are working.
+
+/// This test is taken from the Rust documentation.
+/// https://doc.rust-lang.org/stable/std/sync/struct.Barrier.html
+fn check_barriers() {
+    let mut handles = Vec::with_capacity(10);
+    let barrier = Arc::new(Barrier::new(10));
+    for _ in 0..10 {
+        let c = Arc::clone(&barrier);
+        // The same messages will be printed together.
+        // You will NOT see any interleaving.
+        handles.push(thread::spawn(move || {
+            println!("before wait");
+            c.wait();
+            println!("after wait");
+        }));
+    }
+    for handle in handles {
+        handle.join().unwrap();
+    }
+}
+
+fn main() {
+    check_barriers();
+}

tests/run-pass/concurrency/barrier.stderr

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+warning: thread support is experimental and incomplete: weak memory effects are not emulated.
+

tests/run-pass/concurrency/barrier.stdout

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+before wait
+before wait
+before wait
+before wait
+before wait
+before wait
+before wait
+before wait
+before wait
+before wait
+after wait
+after wait
+after wait
+after wait
+after wait
+after wait
+after wait
+after wait
+after wait
+after wait