Adjust sandbox parts to setup addon test the same way as its postinst…

…all script

scripts/tasks/sandbox-parts.ts

@@ -480,13 +480,22 @@ export async function setupVitest(details: TemplateDetails, options: PassedOptio
     dedent`
       import { defineWorkspace, defaultExclude } from "vitest/config";
       import { storybookTest } from "@storybook/experimental-addon-test/vitest-plugin";
+      import path from 'node:path';
+      import { fileURLToPath } from 'node:url';
+
+      import viteConfig from './vite.config';
+
       ${frameworkPluginImport}
 
+      const dirname =
+        typeof __dirname !== 'undefined' ? __dirname : path.dirname(fileURLToPath(import.meta.url));
+
       export default defineWorkspace([
         {
           ${!isNextjs ? `extends: "${viteConfigPath}",` : ''}
           plugins: [
             storybookTest({
+              configDir: path.join(dirname, '.storybook'),
               storybookScript: "yarn storybook --ci",
               tags: {
                 include: ["vitest"],