maint(ct): simplify test helpers

packages/contracts/contracts/test-helpers/FailingReceiver.sol

@@ -1,13 +1,7 @@
 // SPDX-License-Identifier: MIT
 pragma solidity >=0.8.9;
 
-/**
- * @title FailingReceiver
- */
 contract FailingReceiver {
-    /**
-     * @notice Receiver that always reverts upon receiving ether.
-     */
     receive() external payable {
         require(false, "FailingReceiver");
     }

packages/contracts/contracts/test-helpers/TestERC20.sol

@@ -1,64 +1,12 @@
 // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.9;
 
-// a test ERC20 token with an open mint function
-contract TestERC20 {
-    string public constant name = "Test";
-    string public constant symbol = "TST";
-    uint8 public constant decimals = 18;
-    uint256 public totalSupply;
-    mapping(address => uint256) public balanceOf;
-    mapping(address => mapping(address => uint256)) public allowance;
+import { ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
 
-    event Approval(address indexed owner, address indexed spender, uint256 value);
-    event Transfer(address indexed from, address indexed to, uint256 value);
-
-    constructor() {}
+contract TestERC20 is ERC20 {
+    constructor() ERC20("TEST", "TST") {}
 
     function mint(address to, uint256 value) public {
-        totalSupply = totalSupply + value;
-        balanceOf[to] = balanceOf[to] + value;
-        emit Transfer(address(0), to, value);
-    }
-
-    function _approve(
-        address owner,
-        address spender,
-        uint256 value
-    ) private {
-        allowance[owner][spender] = value;
-        emit Approval(owner, spender, value);
-    }
-
-    function _transfer(
-        address from,
-        address to,
-        uint256 value
-    ) private {
-        balanceOf[from] = balanceOf[from] - value;
-        balanceOf[to] = balanceOf[to] + value;
-        emit Transfer(from, to, value);
-    }
-
-    function approve(address spender, uint256 value) external returns (bool) {
-        _approve(msg.sender, spender, value);
-        return true;
-    }
-
-    function transfer(address to, uint256 value) external returns (bool) {
-        _transfer(msg.sender, to, value);
-        return true;
-    }
-
-    function transferFrom(
-        address from,
-        address to,
-        uint256 value
-    ) external returns (bool) {
-        if (allowance[from][msg.sender] != type(uint256).max) {
-            allowance[from][msg.sender] = allowance[from][msg.sender] - value;
-        }
-        _transfer(from, to, value);
-        return true;
+        _mint(to, value);
     }
 }