Document new getViteConfig() option for Vitest

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

@@ -28,6 +28,19 @@ export default getViteConfig({
 });
 ```
 
+By default, `getViteConfig()` will try to load an Astro config file in your project and apply it to the test environment.
+As of Astro 4.8, if you need to customize the Astro configuration applied in your tests, pass a second argument to `getViteConfig()`:
+
+```js
+export default getViteConfig(
+  { test: { /* Vitest configuration options */ } },
+  {
+    site: 'https://example.com/',
+    trailingSlash: 'always',
+  },
+);
+```
+
 See the [Astro + Vitest starter template](https://github.com/withastro/astro/tree/latest/examples/with-vitest) on GitHub.
 
 ## Cypress