docs: include test env in default env vars (#13708)
docs: include test env in default env vars (#13708)

[Current Environment Variables doc](https://nextjs.org/docs/basic-features/environment-variables#default-environment-variables) mentions `development` and `production` env options but nothing about `test`. Test env is a 3rd valid option, and actually has some implications for `.env.local` from [what I can see in load env configs mechanism](https://github.com/vercel/next.js/blob/canary/packages/next/lib/load-env-config.ts#L68).

This PR aims to include corresponding `test` info for environment variables.
JeromeFitz/next.js,zeit/next.js,flybayer/next.js,flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,JeromeFitz/next.js,azukaru/next.js,zeit/next.js,zeit/next.js,JeromeFitz/next.js,azukaru/next.js,azukaru/next.js,flybayer/next.js,flybayer/next.js