docs: add testing documentation (#1839)

* docs: add test documentation

* docs: update with PR changes

* fix: add missing newline

---------

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Author: MarcL

README.md

@@ -174,6 +174,44 @@ If your site uses pnpm to manage dependencies, currently you must
 public-hoist-pattern[]=*
 ```
 
+## Running the tests
+
+To run the tests, ensure that the dependencies are installed as follows:
+
+```shell
+npm install
+```
+
+Then run the tests:
+
+```shell
+npm test
+```
+
+### End-to-end tests
+
+In order to run the end-to-end (E2E) tests, you'll need to be logged in to Netlify. You can do this using the [Netlify CLI](https://github.com/netlify/cli) with the command:
+
+```shell
+netlify login
+```
+
+Alternatively, you can set an environment variable `NETLIFY_AUTH_TOKEN` to a valid Netlify personal access token. This can be obtained from the [Netlify UI](https://docs.netlify.com/cli/get-started/#obtain-a-token-in-the-netlify-ui).
+
+Then run the E2E tests if logged in:
+
+```shell
+npm run test:next
+```
+
+Or if using an access token:
+
+```shell
+NETLIFY_AUTH_TOKEN=your-token-here npm run test:next
+```
+
+_Note: The E2E tests will be deployed to a Netlify owned site. To deploy to your own site then set the environment variable `NETLIFY_SITE_ID` to your site ID._
+
 ## Feedback
 
 If you think you have found a bug in Next.js on Netlify,