Fix miri test failure

crossbeam-channel/tests/ready.rs

@@ -229,6 +229,7 @@ fn default_when_disconnected() {
 }
 
 #[test]
+#[cfg_attr(miri, ignore)] // this test makes timing assumptions, but Miri is so slow it violates them
 fn default_only() {
     let start = Instant::now();