chore: use helper function for merging vitest config (#8985)

vitest.e2e.config.ts

@@ -1,10 +1,8 @@
-import { UserConfig } from 'vitest/config'
+import { mergeConfig } from 'vitest/config'
 import config from './vitest.config'
 
-export default {
-  ...config,
+export default mergeConfig(config, {
   test: {
-    ...config.test,
     include: ['packages/vue/__tests__/e2e/*.spec.ts']
   }
-} as UserConfig
+})

vitest.unit.config.ts

@@ -1,10 +1,8 @@
-import { UserConfig, configDefaults } from 'vitest/config'
+import { configDefaults, mergeConfig } from 'vitest/config'
 import config from './vitest.config'
 
-export default {
-  ...config,
+export default mergeConfig(config, {
   test: {
-    ...config.test,
     exclude: [...configDefaults.exclude, '**/e2e/**']
   }
-} as UserConfig
+})