Fix miri test failure

Author: taiki-e

crossbeam-channel/tests/zero.rs

@@ -483,6 +483,7 @@ fn fairness() {
 }
 
 #[test]
+#[cfg_attr(miri, ignore)] // Miri is too slow
 fn fairness_duplicates() {
     #[cfg(miri)]
     const COUNT: usize = 50;

crossbeam-utils/tests/wait_group.rs

@@ -36,7 +36,7 @@ fn wait() {
 }
 
 #[test]
-#[cfg(not(miri))] // this test makes timing assumptions, but Miri is so slow it violates them
+#[cfg_attr(miri, ignore)] // this test makes timing assumptions, but Miri is so slow it violates them
 fn wait_and_drop() {
     let wg = WaitGroup::new();
     let (tx, rx) = mpsc::channel();