test(NeptuneCoins): Add #filter for edge cases

Specifically: edge cases can be triggered without i128 overflow but
with numbers exceeding the minimum or maximum number of nau.

src/models/blockchain/type_scripts/neptune_coins.rs

@@ -342,7 +342,8 @@ impl CheckedSub for NeptuneCoins {
 
 impl CheckedAdd for NeptuneCoins {
     /// Return Some(self+other) if (there is no i128-overflow and) the result is
-    /// smaller than the maximum number of nau.
+    /// smaller than or equal to the maximum number of nau, and larger than or
+    /// equal to its negation.
     fn checked_add(&self, v: &Self) -> Option<Self> {
         self.0.checked_add(v.0).and_then(|sum| {
             if !(-Self::MAX_NAU..=Self::MAX_NAU).contains(&sum) {
@@ -785,8 +786,12 @@ pub(crate) mod test {
         a0.checked_add(&a1).unwrap();
     }
 
-    #[proptest]
-    fn checked_add_proptest(lhs: i128, rhs: i128) {
+    #[proptest(cases = 10000)]
+    fn checked_add_proptest(
+        #[filter((#lhs >> 1) + (#rhs >> 1) <= NeptuneCoins::MAX_NAU && (#lhs >> 1) + (#rhs >> 1) >= -NeptuneCoins::MAX_NAU)]
+        lhs: i128,
+        rhs: i128,
+    ) {
         let lhs = NeptuneCoins(lhs >> 1);
         let rhs = NeptuneCoins(rhs >> 1);
         prop_assert!(lhs.checked_add(&rhs).is_some());