feat: gateway test coverage

contracts/test/TestNonStandardERC20.sol

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+
+// solhint-disable no-unused-vars
+
+pragma solidity ^0.8.9;
+
+import { MintableCappedERC20 } from '../MintableCappedERC20.sol';
+
+contract TestNonStandardERC20 is MintableCappedERC20 {
+    bool public shouldFailTransfer;
+
+    constructor(
+        string memory name,
+        string memory symbol,
+        uint8 decimals,
+        uint256 capacity
+    ) MintableCappedERC20(name, symbol, decimals, capacity) {
+        shouldFailTransfer = false;
+    }
+
+    function setFailTransfer(bool _shouldFailTransfer) public {
+        shouldFailTransfer = _shouldFailTransfer;
+    }
+
+    function transfer(address recipient, uint256 amount) public view override returns (bool) {
+        if (shouldFailTransfer) {
+            return false;
+        }
+        revert('Transfer not supported');
+    }
+}