increase size of portable-stories-kitchen-sink to reduce flake:

https://app.circleci.com/pipelines/github/storybookjs/storybook/113893/workflows/052244b4-8873-4b89-806b-885cf66b99a6/jobs/1059649

Author: ndelangen

scripts/ci/test-storybooks.ts

@@ -23,7 +23,7 @@ export function definePortableStoryTest(directory: string) {
     () => ({
       executor: {
         name: 'sb_playwright',
-        class: 'medium',
+        class: 'medium+',
       },
       steps: [
         ...workflow.restoreLinux(),