Miner for ganache-cli. run ganache-cli -d and this will mine blocks at an average of 5 seconds.

miner.js

var ethers = require('ethers');

// ganache deterministic mnemonic
const mnemonic = 'myth like bonus scare over problem client lizard pioneer submit female collect';
// use last funded account
const account = "m/44'/60'/0'/0/9";
// default ganache provider
const provider = new ethers.providers.JsonRpcProvider("http://127.0.0.1:8545");

function getRandomInt(min, max) {
    min = Math.ceil(min);
    max = Math.floor(max);
    return Math.floor(Math.random() * (max - min) + min); //The maximum is exclusive and the minimum is inclusive
}

function timeout(secs) {
    return new Promise(resolve => setTimeout(resolve, secs * 1000));
  }

async function mine() {
    wallet = new ethers.Wallet.fromMnemonic(mnemonic, account);
    wallet = wallet.connect(provider);

    amount = "1";
    nonce = 0;
    while(true){
        console.log('mining tx..');
        const transaction = {
            nonce: nonce++,
            to: wallet.address,
            value: ethers.utils.parseEther(amount),
        };
        await wallet.sendTransaction(transaction);
        console.log('done.');
        await timeout(getRandomInt(2,8)); // average 5 second block time
    }
}
mine();