Remove unused variable warning from tests (#649)

Closes #647
diff --git a/test/test_custom_evented.rs b/test/test_custom_evented.rs
index 25dc747..5a437ca 100644
--- a/test/test_custom_evented.rs
+++ b/test/test_custom_evented.rs
@@ -315,7 +315,7 @@
         let mut final_ready = vec![false; N];
 
 
-        for i in 0..5 {
+        for _ in 0..5 {
             poll.poll(&mut events, None).unwrap();
 
             for event in &events {