Added one additional BigInt=>u128 test.

chain/rust/src/utils.rs

@@ -1049,7 +1049,7 @@ mod tests {
 
     #[test]
     fn bigint_uint_u128_roundtrip() {
-        let int = u64::MAX;
+        let int = 462_164_030_739_157_517;
         let x = BigInteger::from_int(&Int::Uint {
             value: int,
             encoding: None,
@@ -1067,6 +1067,16 @@ mod tests {
         assert_eq!(x.as_u128(), Some(int as u128))
     }
 
+    #[test]
+    fn bigint_uint_u128_roundtrip_max() {
+        let int = u64::MAX;
+        let x = BigInteger::from_int(&Int::Uint {
+            value: int,
+            encoding: None,
+        });
+        assert_eq!(x.as_u128(), Some(int as u128))
+    }
+
     #[test]
     fn bigint_uint_u128_min() {
         let bytes = [0x00];