give title to a code block

Co-authored-by: Chris Swithinbank <swithinbank@gmail.com>

src/content/docs/en/guides/testing.mdx

@@ -52,7 +52,7 @@ See the [Astro + Vitest starter template](https://github.com/withastro/astro/tre
 
 You can natively test Astro components using the [container API](/en/reference/container-reference/). First, setup [`vitest` as explained above](#vitest), then create a `.test.js` file to test your component:
 
-```js
+```js title="example.test.js"
 import { experimental_AstroContainer as AstroContainer } from 'astro/container';
 import { expect, test } from 'vitest';
 import Card from '../src/components/Card.astro';