Add ops unit tests for Weight

Copy the unit tests from `FeeRate` that prove `Add`, `AddAssign`, `Sub`,
and `SubAssign` for references.

units/src/weight.rs

@@ -357,4 +357,52 @@ mod tests {
 
     #[test]
     fn checked_div_overflows() { assert!(TWO.checked_div(0).is_none()) }
+
+    #[test]
+    #[allow(clippy::op_ref)]
+    fn addition() {
+        let one = Weight(1);
+        let two = Weight(2);
+        let three = Weight(3);
+
+        assert!(one + two == three);
+        assert!(&one + two == three);
+        assert!(one + &two == three);
+        assert!(&one + &two == three);
+    }
+
+    #[test]
+    #[allow(clippy::op_ref)]
+    fn subtract() {
+        let one = Weight(1);
+        let two = Weight(2);
+        let three = Weight(3);
+
+        assert!(three - two == one);
+        assert!(&three - two == one);
+        assert!(three - &two == one);
+        assert!(&three - &two == one);
+    }
+
+    #[test]
+    fn add_assign() {
+        let mut f = Weight(1);
+        f += Weight(2);
+        assert_eq!(f, Weight(3));
+
+        let mut f = Weight(1);
+        f += &Weight(2);
+        assert_eq!(f, Weight(3));
+    }
+
+    #[test]
+    fn sub_assign() {
+        let mut f = Weight(3);
+        f -= Weight(2);
+        assert_eq!(f, Weight(1));
+
+        let mut f = Weight(3);
+        f -= &Weight(2);
+        assert_eq!(f, Weight(1));
+    }
 }