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.

docs/basic-features/environment-variables.md

@@ -93,3 +93,13 @@ If you've configured [Development Environment Variables](https://vercel.com/docs
 ```bash
 vercel env pull .env.local
 ```
+
+## Test Environment Variables
+
+Apart from `development` and `production` environments, there is a 3rd option available: `test`. In the same way you can set defaults for development or production environments, you can do the same with `.env.test` file for testing environment (though this one is not so common as the previous two).
+
+This one is useful when running tests with tools like `jest` or `cypress` where you need to set specific environment vars only for testing purposes. Test default values will be loaded if `NODE_ENV` is set to `test`, though you usually don't need to do this manually as testing tools will address it for you.
+
+There is a small difference between `test` environment, and both `development` and `production` that you need to bear in mind: `.env.local` won't be loaded, as you expect tests to produce the same results for everyone. This way every test execution will use same env defaults across different executions by ignoring your `.env.local` (which is intended to override the default set).
+
+> **Note**: similar to Default Environment Variables, `.env.test` file should be included in your repository, but `.env.test.local` shouldn't, as `.env*.local` are intended to be ignored through `.gitignore`.