Give slowest rendering fixtures tests (images) more time (#43787)

tests/rendering-fixtures/images.js

@@ -1,9 +1,14 @@
+import { jest } from '@jest/globals'
 import sharp from 'sharp'
 
 import { get, head, getDOM } from '../../tests/helpers/e2etest.js'
 import { MAX_WIDTH } from '#src/content-render/unified/rewrite-asset-img-tags.js'
 
 describe('render Markdown image tags', () => {
+  // The default timeout is 5 seconds, which is not enough for this test
+  // in potentially slower CI environments.
+  jest.setTimeout(30 * 1000)
+
   test('page with a single image', async () => {
     const $ = await getDOM('/get-started/images/single-image')