fix: Add Hardhat test suite for ListingRegistry.sol

[Xanoutas]

Fix for #3: Add Hardhat test suite for ListingRegistry.sol

// contracts/ListingRegistry.sol
pragma solidity ^0.8.0;

import "@openzeppelin/contracts/utils/cryptography/ECDSA.sol";

contract ListingRegistry {
    mapping(address => bool) public sellers;
    mapping(uint256 => string) public listingMetadata;

    function isValidSignature(bytes32 hash, bytes memory signature) public view returns (bool) {
        address signer = ECDSA.recover(hash, signature);
        return sellers[signer];
    }

    function addSeller(address seller) public {
        sellers[seller] = true;
    }

    function createListing(uint256 listingId, string memory metadata, bytes memory signature) public {
        require(isValidSignature(keccak256(abi.encodePacked(listingId, metadata)), signature), "Invalid signature");
        listingMetadata[listingId] = metadata;
    }
}

// test/ListingRegistry.test.js
const { expect } = require("chai");
const { ethers } = require("hardhat");

describe("ListingRegistry", function () {
    let listingRegistry;
    let owner;
    let seller;
    let nonSeller;

    beforeEach(async function () {
        [owner, seller, nonSeller] = await ethers.getSigners();
        const ListingRegistry = await ethers.getContractFactory("ListingRegistry");
        listingRegistry = await ListingRegistry.deploy();
        await listingRegistry.deployed();
    });

    it("should validate valid signature", async function () {
        const listingId = 1;
        const metadata = "Sample metadata";
        const hash = ethers.utils.keccak256(ethers.utils.encodePacked(listingId, metadata));
        const signature = await seller.signMessage(ethers.utils.arrayify(hash));

        await listingRegistry.addSeller(seller.address);
        await listingRegistry.createListing(listingId, metadata, signature);

        expect(await listingRegistry.listingMetadata(listingId)).to.equal(metadata);
    });

    it("should reject invalid signature", async function () {
        const listingId = 2;
        const metadata = "Sample metadata";
        const hash = ethers.utils.keccak256(ethers.utils.encodePacked(listingId, metadata));
        const signature = await nonSeller.signMessage(ethers.utils.arrayify(hash));

        await listingRegistry.addSeller(seller.address);
        await expect(listingRegistry.createListing(listingId, metadata, signature)).to.be.revertedWith("Invalid signature");
    });

    it("should emit event on listing creation", async function () {
        const listingId = 3;
        const metadata = "Sample metadata";
        const hash = ethers.utils.keccak256(ethers.utils.encodePacked(listingId, metadata));
        const signature = await seller.signMessage(ethers.utils.arrayify(hash));

        await listingRegistry.addSeller(seller.address);
        await expect(listingRegistry.createListing(listingId, metadata, signature))
            .to.emit(listingRegistry, "ListingCreated")
            .withArgs(listingId, metadata);
    });

    it("should accurately map seller 

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Closes #3