Some test coverage for sign handling

src/bigint.rs

@@ -479,6 +479,18 @@ mod tests {
         let sum = a + b;
         assert_eq!(sum.inner[0], 0usize);
         assert_eq!(sum.inner[1], 1usize);
+
+        let a = BigInt::from(10);
+        let b = -BigInt::from(5);
+        let sum = a + b;
+        assert_eq!(sum.inner[0], 5usize);
+        assert_eq!(sum.sign, Positive);
+
+        let a = -BigInt::from(5);
+        let b = BigInt::from(10);
+        let sum = a + b;
+        assert_eq!(sum.inner[0], 5usize);
+        assert_eq!(sum.sign, Positive);
     }
 
     #[test]
@@ -507,6 +519,18 @@ mod tests {
             core::usize::MAX - core::u16::MAX as usize
         );
         assert_eq!(diff.inner.len(), 1);
+
+        let a = BigInt::from(5);
+        let b = -BigInt::from(3);
+        let diff = a - b;
+        assert_eq!(diff.sign, Positive);
+        assert_eq!(diff.inner[0], 8usize);
+
+        let a = -BigInt::from(5);
+        let b = BigInt::from(3);
+        let diff = a - b;
+        assert_eq!(diff.sign, Negative);
+        assert_eq!(diff.inner[0], 8usize);
     }
 
     #[test]