refactor: surplus MATIC test

The test was failing because it wasn't sending enough MATIC to OffsetHelper.

I started using `howMuchETHShouldISendToSwap` in the test which fixed it, but I realised the error message from Sushi is not useful.

So I implemented a check with a useful message within the OffsetHelper contract for when someone doesn't send enough ETH.

contracts/OffsetHelper.sol

@@ -215,6 +215,16 @@ contract OffsetHelper is OffsetHelperStorage {
         path[1] = eligibleTokenAddresses["USDC"];
         path[2] = _toToken;
 
+        uint256[] memory expectedAmounts = routerSushi.getAmountsIn(
+            _amount,
+            path
+        );
+
+        require(
+            msg.value >= expectedAmounts[0],
+            "You didn't send enough MATIC."
+        );
+
         // swap MATIC for tokens
         uint256[] memory amounts = routerSushi.swapETHForExactTokens{
             value: msg.value
@@ -225,7 +235,8 @@ contract OffsetHelper is OffsetHelperStorage {
             (bool success, ) = msg.sender.call{value: leftoverETH}(
                 new bytes(0)
             );
-            require(success, "Failed to send dust ETH back to user.");
+
+            require(success, "Failed to send surplus ETH back to user.");
         }
 
         // update balances

test/OffsetHelper.ts

@@ -651,12 +651,17 @@ describe("Offset Helper - autoOffset", function () {
         offsetHelper.address
       );
 
+      const maticToSend = await offsetHelper.howMuchETHShouldISendToSwap(
+        addresses.nct,
+        parseEther("1.0")
+      );
+
       await (
         await offsetHelper["swap(address,uint256)"](
           addresses.nct,
           parseEther("1.0"),
           {
-            value: parseEther("5.0"),
+            value: maticToSend.add(parseEther("0.5")),
           }
         )
       ).wait();