test flash borrowing on flashpot - just send a fee's worth of ETH to this address then call flashBorrow (using 0 address for 'address')

FlashPotBorrow.sol

// SPDX-License-Identifier: GPL-3.0-or-later

pragma solidity >=0.8.4;

import "https://github.com/Rari-Capital/solmate/blob/audit-fixes/src/utils/SafeTransferLib.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/interfaces/IERC3156FlashLender.sol";

contract FlashPotBorrow is IERC3156FlashBorrower {
    using SafeTransferLib for address;

    enum Action {NORMAL, OTHER}

    IERC3156FlashLender lender;

    constructor (
        IERC3156FlashLender lender_
    ) {
        lender = lender_;
    }

    /// @dev ERC-3156 Flash loan callback
    function onFlashLoan(
        address initiator,
        address,
        uint256 amount,
        uint256 fee,
        bytes calldata data
    ) public override returns(bytes32) {
        uint256 _repayment = amount + fee;
        require(
            msg.sender == address(lender),
            "FlashBorrower: Untrusted lender"
        );
        require(
            initiator == address(this),
            "FlashBorrower: Untrusted loan initiator"
        );
        (Action action) = abi.decode(data, (Action));
        if (action == Action.NORMAL) {
            address(lender).safeTransferETH(_repayment);
        } else if (action == Action.OTHER) {
            // do another
        }
        return keccak256("ERC3156FlashBorrower.onFlashLoan");
    }

    /// @dev Initiate a flash loan
    function flashBorrow(
        address,
        uint256 amount
    ) public {
        bytes memory data = abi.encode(Action.NORMAL);
        lender.flashLoan(IERC3156FlashBorrower(this), address(0), amount, data);
    }
}