Test for deposit from another erc20 contract to my contract, how do it right?

I'm trying add deposit from one mock contract to my contract, but balance in my contract is zero. How I can do it right? Below is my code:

// imports
const { ethers, waffle } = require('hardhat')
const { deployContract } = waffle
// then
it('deposit USDT', async function () {
    const [walletUSDT, walletMatrix] = await ethers.getSigners()
    const tokenUSDT = await deployContract(walletUSDT, MockUSDT);
    const tokenMatrix = await deployContract(walletMatrix, MXToken);
    tokenUSDT.transfer(walletMatrix.address, 70)
    const test = await tokenMatrix.balanceOf(walletMatrix.address)