feat: add tests for withdraw, fee and make it DRY

foundry/test/mock/MockERC20.sol

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: Unlicense
+pragma solidity ^0.8.28;
+
+import {ERC20} from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+
+contract MockERC20 is ERC20 {
+    constructor(string memory name_, string memory symbol_)
+        ERC20(name_, symbol_)
+    {}
+
+    function mint(address to, uint256 amount) external {
+        _mint(to, amount);
+    }
+
+    function decimals() public view virtual override returns (uint8) {
+        return 18;
+    }
+}