build: disable in-suite parallelization

There appears to still be some flakiness that I cannot reproduce when running `.only` on a test,
which implies that there's somethhing conflicting somehow across tests running concurrently.

Author: serhalp

playwright.config.ts

@@ -5,9 +5,7 @@ import { defineConfig } from '@playwright/test'
  */
 export default defineConfig({
   testDir: './tests/e2e',
-  /* Run tests in files in parallel */
-  fullyParallel: true,
-  /* Fail the build on CI if you accidentally left test.only in the source code. */
+ /* Fail the build on CI if you accidentally left test.only in the source code. */
   forbidOnly: !!process.env.CI,
   /* Retry on CI only */
   retries: process.env.CI ? 2 : 0,