docs: update vitest config to match vitest docs (#3037)

* Update testing.md

in vite,
mergeConfig accepts only config in object form. If you have a config in callback form, you should call it before passing into mergeConfig.

You can use the defineConfig helper to merge a config in callback form with another config:

https://vite.dev/guide/api-javascript.html#mergeconfig

* Fix formatting

---------

Co-authored-by: Danilo Britto <dbritto.dev@gmail.com>

docs/guides/testing.md

@@ -274,8 +274,9 @@ vi.mock('zustand') // to make it work like Jest (auto-mocking)
 import { defineConfig, mergeConfig } from 'vitest/config'
 import viteConfig from './vite.config'
 
-export default mergeConfig(
-  viteConfig,
+export default defineConfig((configEnv) =>
+  mergeConfig(
+    viteConfig(configEnv),
     defineConfig({
       test: {
         globals: true,
@@ -283,6 +284,7 @@ export default mergeConfig(
         setupFiles: ['./setup-vitest.ts'],
       },
     }),
+  ),
 )
 ```