std: Add debugging for a failing test on appveyor

I'm not sure why this is failing, so let's hopefully get some more
information to help investigation!

src/libstd/time.rs

@@ -481,7 +481,8 @@ mod tests {
             let (a, b) = ($a, $b);
             if a != b {
                 let (a, b) = if a > b {(a, b)} else {(b, a)};
-                assert!(a - Duration::new(0, 100) <= b);
+                assert!(a - Duration::new(0, 100) <= b,
+                        "{:?} is not almost equal to {:?}", a, b);
             }
         })
     }