frontend: use import instead of manually extending expect in src/test…

…/setup.js

See release notes at https://github.com/testing-library/jest-dom/releases/tag/v6.0.0

frontend/tests/setup.js

@@ -1,12 +1,9 @@
 import { expect, afterEach } from 'vitest'
 import { cleanup } from '@testing-library/vue'
-import matchers from '@testing-library/jest-dom/matchers'
+import '@testing-library/jest-dom/vitest'
 import snapshotSerializer from 'jest-serializer-vue-tjw'
 import 'vitest-canvas-mock'
 
-// extends Vitest's expect method with methods from vue-testing-library
-expect.extend(matchers)
-
 // runs a cleanup after each test case (e.g. clearing jsdom)
 afterEach(() => {
   cleanup()