Add TierLimits unit tests (#12)

* Add TierLimits unit tests

smart_contract/src/karma_tiers.rs

@@ -1,4 +1,5 @@
 use std::collections::BTreeMap;
+use std::ops::Add;
 // third-party
 use alloy::{
     primitives::{Address, U256},
@@ -67,6 +68,14 @@ impl From<&TierIndex> for u8 {
     }
 }
 
+impl Add<u8> for TierIndex {
+    type Output = TierIndex;
+
+    fn add(self, rhs: u8) -> Self::Output {
+        Self(self.0 + rhs)
+    }
+}
+
 #[derive(Debug, Clone, PartialEq)]
 pub struct Tier {
     pub min_karma: U256,