Running and writing tests

Get dotenv
Run yarn test

If you want to write a test with a specific network in mind do the following

import { setFork } from "hardhat";
describe("Foo", function () {
    before(async () => {
        // If you want to set the network for the scope of this test file
        await setFork("short name (eth, bsc, avax, etc)");
        // after this promise resolves ethers.provider will be forking the network, so you just go on as normal
    });
});

Note: Try to avoid switching networks in the test multiple times, since it will destroy all the EVM state modification that you're did

So as the rule of thumb do the setFork BEFORE you deploy your contracts