Test amount ops that involve an integer

From the amount types `Div`, `Mul`, and `Rem` can have an integer on one
side. Test them - includes commented out test for one missing combo.

units/src/amount/tests.rs

@@ -1207,7 +1207,10 @@ fn signed_addition() {
     assert_eq!(ssat(-307) + ssat(461), NumOpResult::from(ssat(154)));
     assert_eq!(ssat(307) + ssat(-461), NumOpResult::from(ssat(-154)));
     assert_eq!(ssat(-307) + ssat(-461), NumOpResult::from(ssat(-768)));
-    assert_eq!(SignedAmount::MAX_MONEY + -SignedAmount::MAX_MONEY, NumOpResult::from(SignedAmount::ZERO));
+    assert_eq!(
+        SignedAmount::MAX_MONEY + -SignedAmount::MAX_MONEY,
+        NumOpResult::from(SignedAmount::ZERO)
+    );
 }
 
 #[test]
@@ -1236,6 +1239,47 @@ fn signed_subtraction() {
     assert_eq!(ssat(-307) - ssat(-461), NumOpResult::from(ssat(154)));
 }
 
+#[test]
+fn op_int_combos() {
+    let sat = Amount::from_sat;
+    let ssat = SignedAmount::from_sat;
+
+    let res = |sat| NumOpResult::from(Amount::from_sat(sat));
+    let sres = |ssat| NumOpResult::from(SignedAmount::from_sat(ssat));
+
+    assert_eq!(sat(23) * 31, res(713));
+    assert_eq!(ssat(23) * 31, sres(713));
+    assert_eq!(res(23) * 31, res(713));
+    assert_eq!(sres(23) * 31, sres(713));
+
+    // assert_eq!(31 * sat(23), res(713));
+    // assert_eq!(31 * ssat(23), sres(713));
+
+    // No remainder.
+    assert_eq!(sat(1897) / 7, res(271));
+    assert_eq!(ssat(1897) / 7, sres(271));
+    assert_eq!(res(1897) / 7, res(271));
+    assert_eq!(sres(1897) / 7, sres(271));
+
+    // Truncation works as expected.
+    assert_eq!(sat(1901) / 7, res(271));
+    assert_eq!(ssat(1901) / 7, sres(271));
+    assert_eq!(res(1901) / 7, res(271));
+    assert_eq!(sres(1901) / 7, sres(271));
+
+    // No remainder.
+    assert_eq!(sat(1897) % 7, res(0));
+    assert_eq!(ssat(1897) % 7, sres(0));
+    assert_eq!(res(1897) % 7, res(0));
+    assert_eq!(sres(1897) % 7, sres(0));
+
+    // Remainder works as expected.
+    assert_eq!(sat(1901) % 7, res(4));
+    assert_eq!(ssat(1901) % 7, sres(4));
+    assert_eq!(res(1901) % 7, res(4));
+    assert_eq!(sres(1901) % 7, sres(4));
+}
+
 #[test]
 fn check_const() {
     assert_eq!(SignedAmount::ONE_BTC.to_sat(), 100_000_000);