Hardhat Testing – How to Fund Contract with ERC20 Tokens in Hardhat Test

I have a contract I'm testing using hardhat.

What is a good approach to fund that contract with an arbitrary ERC20 token (WETH in this case)?

I am testing using a hardhat mainnet fork, my approach is to impersonate an account which has lots of that token and send from that account to my contract; This is not working, seems like I am not successful in impersonating that account. Also, it seems like there might be a better way.

MyTest.js

const { expect } = require("chai");
const { ethers } = require("hardhat");

const IERC20_SOURCE = "@openzeppelin/contracts/token/ERC20/IERC20.sol:IERC20";

const WETH_WHALE = "0xF04a5cC80B1E94C69B48f5ee68a08CD2F09A7c3E";
const WETH_ADDRESS = "0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2";


describe("Test Contract", async () => {

  let contractToTest, wethContract;
  let admin;

  beforeEach(async () => {
        ([admin] = await ethers.getSigners());

        // Deploy contract
        const MyContract = await ethers.getContractFactory("MyContract");
        contractToTest = await MyContract.deploy();
        await contractToTest.deployed();
       
        // ~~~~~~~~~~~~~ HERE LIES THE ISSUE ~~~~~~~~~~~~

        // Send some ERC20 to my contract
        // by trying to impersonate a whale and sending stuff from their accounts
        await hre.network.provider.request({
            method: "hardhat_impersonateAccount",
            params: [WETH_WHALE],
        });
        const signer = await ethers.provider.getSigner(WETH_WHALE);
        signer.address = signer._address;
        wethContract = await hre.ethers.getContractAt(IERC20_SOURCE, WETH_ADDRESS, signer);
        wethContract = wethContract.connect(signer);

        console.log("Signer WETH balance", await wethContract.balanceOf(signer.address))
        // Signer WETH balance BigNumber { value: "480881976480357719815777" }

        // ~~~~~~~~~~~~~~~ ERROR AFTER THIS LINE :
        await wethContract.transfer(contractToTest.address, ethers.utils.parseEther("10"));
        //  InvalidInputError: sender doesn't have enough funds to send tx. 
        // The max upfront cost is: 3311216806566865328 and the sender's account only has: 0

        // WE DON'T GET HERE: 
        // check that contractToTest actually holds the 10 WETH
        expect(await wethContract.balanceOf(contractToTest.address)).to.equal(
            ethers.utils.parseEther("10"), 
            "Contract to test should hold 10 WETH!");
  });

});

hardhat.config.js

require("@nomiclabs/hardhat-waffle");
require('dotenv').config();
require("@nomiclabs/hardhat-ethers");

/**
 * @type import('hardhat/config').HardhatUserConfig
 */
const { ALCHEMY_MAINNET_URL, PRIVATE_KEY } = process.env;
module.exports = {
  solidity: "0.6.12",
  defaultNetwork: "hardhat",
  networks: {
      hardhat: {
        forking: {
          url: `https://eth-mainnet.alchemyapi.io/v2/${ALCHEMY_MAINNET_URL}`,
          // accounts: [`0x${PRIVATE_KEY}`]
        }
      }
  },
};