Add note for loading env files during testing (#22982)

This adds a note to the environment variables documentation to mention how the env files can be loaded using the `@next/env` package as this has been brought up a few times. 

x-ref: https://github.com/vercel/next.js/issues/22936#issuecomment-796665303
x-ref: https://github.com/vercel/next.js/issues/17903

docs/basic-features/environment-variables.md

@@ -135,3 +135,15 @@ This one is useful when running tests with tools like `jest` or `cypress` where
 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`.
+
+While running unit tests you can make sure to load your environment variables the same way Next.js does by leveraging the `loadEnvConfig` function from the `@next/env` package.
+
+```js
+// The below can be used in a Jest global setup file or similar for your testing set-up
+import { loadEnvConfig } from '@next/env'
+
+export default async () => {
+  const projectDir = process.cwd()
+  loadEnvConfig(projectDir)
+}
+```