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

@@ -274,15 +274,17 @@ 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(
+export default defineConfig((configEnv) =>
-  viteConfig,
+  mergeConfig(
-  defineConfig({
+    viteConfig(configEnv),
-    test: {
+    defineConfig({
-      globals: true,
+      test: {
-      environment: 'jsdom',
+        globals: true,
-      setupFiles: ['./setup-vitest.ts'],
+        environment: 'jsdom',
-    },
+        setupFiles: ['./setup-vitest.ts'],
-  }),
+      },
+    }),
+  ),
 )
 ```