Fix clippy::match_like_matches_macro warning in test

```
warning: match expression looks like `matches!` macro
  --> tests/barrier.rs:32:21
   |
32 |               assert!(match res {
   |  _____________________^
33 | |                 Err(_err) => true,
34 | |                 _ => false,
35 | |             });
   | |_____________^ help: try this: `matches!(res, Err(_err))`
   |
   = note: `#[warn(clippy::match_like_matches_macro)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_like_matches_macro
```

Author: taiki-e

tests/barrier.rs

@@ -29,10 +29,7 @@ fn smoke() {
             // At this point, all spawned threads should be blocked,
             // so we shouldn't get anything from the cahnnel.
             let res = rx.try_recv();
-            assert!(match res {
-                Err(_err) => true,
-                _ => false,
-            });
+            assert!(res.is_err());
 
             let mut leader_found = barrier.wait().await.is_leader();