Add guide for migrating from mocha to node:test

[fvictorio]

We should have an optional guide in the "Migrate from Hardhat 2" docs that explains how to migrate from mocha to node:test. It doesn't need to be comprehensive, and ideally we link to some external resource that goes in more depth.

We should include a link to it somewhere in the main migration guide, to nudge people to migrate from mocha. For example, a tip in the "Migrating tests" section could work.