Rename iter len unit test

Rust convention is to not use `test_` prefix on unit tests. Also this
unit test is testing that the `ExactSizedIterator` trait is implemented
and working.

Re-name unit test to `exact_sized_iterator`.

bitcoin/src/blockdata/witness.rs

@@ -757,7 +757,7 @@ mod test {
     }
 
     #[test]
-    fn test_iter_len() {
+    fn exact_sized_iterator() {
         let mut witness = Witness::default();
         for i in 0..5 {
             assert_eq!(witness.iter().len(), i);