fix(testing): add note about running test setup in serial mode (#1798)

docs/testing/playwright/overview.mdx

@@ -49,6 +49,9 @@ description: Use Playwright to write end-to-end tests with Clerk.
   import { clerkSetup } from '@clerk/testing/playwright'
   import { test as setup } from '@playwright/test'
 
+  // Setup must be run serially, this is necessary if Playwright is configured to run fully parallel: https://playwright.dev/docs/test-parallel
+  setup.describe.configure({ mode: 'serial' })
+
   setup('global setup', async ({}) => {
     await clerkSetup()
   })