Assert error type

Improve test granularity by asserting the specific type of overflow
returned.

Author: yancyribbens

units/src/fee.rs

@@ -344,8 +344,11 @@ mod tests {
 
     #[test]
     fn fee_wu() {
-        let fee_overflow = FeeRate::from_sat_per_kwu(10).to_fee(Weight::MAX);
-        assert!(fee_overflow.is_error());
+        let operation = FeeRate::from_sat_per_kwu(10)
+            .to_fee(Weight::MAX)
+            .unwrap_err()
+            .operation();
+        assert!(operation.is_multiplication());
 
         let fee_rate = FeeRate::from_sat_per_vb(2).unwrap();
         let weight = Weight::from_vb(3).unwrap();