Add env section to testing guide

[drwpow]

Suggestion

I had a little bit of trouble getting a good testing setup for Hono + Cloudflare Workers, because they use so many env vars. I read this testing guide initially, but as it gave no hints to env, I tried a bunch of different solutions (even including some complicated Vitest code replacement) before I realized this was so easy to pull off.

Open to feedback/suggestions if there’s a better way to do this (i.e. if there’s a better recommended way), but I found this a relatively simple, flexible setup for my own testing.

[yusukebe]

Hi @drwpow

This is a good suggestion.

For mocking, this approach is great. I'll merge this for it.

And, another approach, we can use Wrangler's unstable_dev, but it's not documented. The issue: #194