sync: add loom test for mpsc (#903)

This patch updates tokio_sync::mpsc to support using loom for fuzz
testing. It includes a basic fuzz test.

tokio-sync/tests/fuzz_mpsc.rs

@@ -0,0 +1,39 @@
+extern crate futures;
+#[macro_use]
+extern crate loom;
+
+macro_rules! if_fuzz {
+    ($($t:tt)*) => {
+        $($t)*
+    }
+}
+
+#[path = "../src/mpsc/mod.rs"]
+#[allow(warnings)]
+mod mpsc;
+
+#[path = "../src/semaphore.rs"]
+#[allow(warnings)]
+mod semaphore;
+
+use futures::{Stream, future::poll_fn};
+use loom::futures::block_on;
+use loom::thread;
+
+#[test]
+fn closing_tx() {
+    loom::fuzz(|| {
+        let (mut tx, mut rx) = mpsc::channel(16);
+
+        thread::spawn(move || {
+            tx.try_send(()).unwrap();
+            drop(tx);
+        });
+
+        let v = block_on(poll_fn(|| rx.poll())).unwrap();
+        assert!(v.is_some());
+
+        let v = block_on(poll_fn(|| rx.poll())).unwrap();
+        assert!(v.is_none());
+    });
+}