Add sanity-check unit test for RecoveryId

This is kinda silly but gets me 100% coverage from kcov

src/lib.rs

@@ -506,6 +506,12 @@ mod tests {
         assert_eq!(pk, new_pk);
     }
 
+    #[test]
+    fn recid_sanity_check() {
+        let one = RecoveryId(1);
+        assert_eq!(one, one.clone());
+    }
+
     #[test]
     fn invalid_pubkey() {
         let s = Secp256k1::new();