Add chapter to "Testing" section about writing front-end test and DOM integration

[bartlomieju]

This question comes up fairly often on Discord, and here's latest question on GitHub: https://github.com/denoland/deno/issues/14565

Even though we have a chapter about using DOM in Deno, we should update "Testing" chapter to provide some "common recipes" for testing frontend code. We might even want to consider providing some simpler integration in deno_std/testing

[RobertAKARobin]

This would be great! Thanks!