frontend: run tests in ci in single thread

CI sometimes still hangs, trying to run without threads

- vitest-dev/vitest#2008
- vitest-dev/vitest#3077

In earlier vitest versions the optinon was --no-threads, but this
changed to --pool=forks in:
- https://github.com/vitest-dev/vitest/releases/tag/v1.0.0-beta.0
- https://github.com/vitest-dev/vitest/blob/main/docs/guide/migration.md#pools-are-standardized-4172

Changed vitest config, so that CI and make webtest are both using
a single thread.

frontends/web/vite.config.mjs

@@ -24,6 +24,7 @@ export default defineConfig(() => {
       css: false,
       environment: 'jsdom',
       globals: true,
+      pool: 'forks',
       setupFiles: './vite.setup-tests.mjs',
     },
   };