Run HTML validation with id="content" wrapper

We previously enabled the rule `no-missing-references` but lots of examples target `id="content"` on the preview container which isn’t available when only the component is rendered

For example from fieldset.yaml

```
  - name: with describedBy
    hidden: true
    options:
      describedBy: content
      legend:
        text: Which option?
```

packages/govuk-frontend/src/govuk/components/components.template.test.js

@@ -9,6 +9,7 @@ const {
 } = require('@govuk-frontend/lib/components')
 const validatorConfig = require('govuk-frontend/.htmlvalidate.js')
 const { HtmlValidate } = require('html-validate')
+const { outdent } = require('outdent')
 
 describe('Components', () => {
   let nunjucksEnvCustom
@@ -65,10 +66,14 @@ describe('Components', () => {
       // Validate component examples
       for (const { component: componentName, fixtures } of componentsFixtures) {
         const fixtureTasks = fixtures.map(async (fixture) => {
-          const html = render(componentName, {
-            context: fixture.options,
-            fixture
-          })
+          const html = outdent`
+            <div id="content" class="govuk-width-container">
+              ${render(componentName, {
+                context: fixture.options,
+                fixture
+              })}
+            </div>
+          `
 
           // Validate HTML
           return expect({