Update test docs for Puppeteer browser inspection

Includes tip regarding `await jestPuppeteer.debug()` with ESLint config changes to allow `jestPuppeteer` as a known global

docs/releasing/testing-and-linting.md

@@ -76,6 +76,8 @@ We write functional tests for every component to check the output of our Nunjuck
 
 If a component uses JavaScript, we also write functional tests in a `[component name].test.js` file, for example [checkboxes.test.js](../../src/govuk/components/checkboxes/checkboxes.test.js). These component tests check that interactions, such as a mouse click, have the expected result.
 
+When running tests in [headless Chrome](https://developers.google.com/web/updates/2017/04/headless-chrome) it's useful to configure [`{ headless: false }`](../../jest-puppeteer.config.js) and run `await jestPuppeteer.debug()` from the test. This pauses an open browser for inspection.
+
 You should also test component Javascript logic with unit tests, in a `[component name].unit.test.mjs` file. These tests are better suited for testing behind-the-scenes logic, or in cases where the final output of some logic is not a change to the component markup.
 
 ### Global tests

jest-puppeteer.config.js

@@ -30,6 +30,7 @@ module.exports = {
        */
       '--no-startup-window'
     ],
+    headless: false,
     waitForInitialPage: false
   },
 

src/.eslintrc.js

@@ -46,7 +46,7 @@ module.exports = {
     },
     {
       // Matches 'JavaScript component tests' in jest.config.mjs
-      // to ignore unknown 'page' and 'browser' Puppeteer globals
+      // to ignore unknown Jest Puppeteer globals
       files: [
         '**/components/globals.test.mjs',
         '**/components/*/*.test.{js,mjs}'
@@ -56,8 +56,9 @@ module.exports = {
         '**/*.unit.test.{js,mjs}'
       ],
       globals: {
-        page: true,
-        browser: true
+        page: 'readonly',
+        browser: 'readonly',
+        jestPuppeteer: 'readonly'
       }
     }
   ]