Merge rust-bitcoin#4648: units: Test generic Add and Sub

a520a8a units: Test generic Add and Sub (Tobin C. Harding)

Pull request description:

  We have `Add` and `Sub` implemented for `NumOpResult<T>` but because the impls are generic they are not grouped in the file with the other add/sub impls. This makes it hard to see if they are supported.
  
  Add to the `add` and `sub` unit tests to verify support is implemented.


ACKs for top commit:
  apoelstra:
    ACK a520a8a; successfully ran local tests


Tree-SHA512: 01a10e03f759942910c80d33e0150633415bc678c0ed5a7a403265226418a3594a658dff37ec770de3409a785e4d80213352075f6cca9ccbe3baf827e63b35ad

Author: apoelstra

units/src/amount/tests.rs

@@ -227,6 +227,10 @@ fn add() {
     assert!(ssat(-127) + ssat(179) == ssat(52).into());
     assert!(ssat(127) + ssat(-179) == ssat(-52).into());
     assert!(ssat(-127) + ssat(-179) == ssat(-306).into());
+
+    // Implemented using generic impl.
+    assert!(res(127) + sat(179) == sat(306).into());
+    assert!(sres(127) + ssat(179) == ssat(306).into());
 }
 
 #[test]
@@ -240,6 +244,10 @@ fn sub() {
     assert!(ssat(-127) - ssat(179) == ssat(-306).into());
     assert!(ssat(127) - ssat(-179) == ssat(306).into());
     assert!(ssat(-127) - ssat(-179) == ssat(52).into());
+
+    // Implemented using generic impl.
+    assert!(res(179) - sat(127) == sat(52).into());
+    assert!(sres(179) - ssat(127) == ssat(52).into());
 }
 
 #[test]