[ci] Speed up package tests by enabling parallel execution (#13601)

Author: petebacondarwin

.github/workflows/test-and-check.yml

@@ -140,7 +140,8 @@ jobs:
         if: steps.changes.outputs.everything_but_markdown == 'true' && matrix.suite == 'packages-and-tools'
         # We skip @cloudflare/vitest-pool-workers tests in CI on Windows because they're very flaky. We still run the vitest-pool-workers-examples fixture, which is a comprehensive set of example tests and gives us a lot of confidence.
         # The @cloudflare/vitest-pool-workers tests skipped are things like watch mode, which constantly times out probably due to the github runners in use.
-        run: pnpm run test:ci --log-order=stream --concurrency=1 --filter="./packages/*" ${{ matrix.os == 'windows-latest' && '--filter="!./packages/vitest-pool-workers"' || '' }}
+        # Package tests are well-isolated (separate processes, temp dirs, random ports, MSW mocks) and can safely run in parallel.
+        run: pnpm run test:ci --log-order=stream --filter="./packages/*" ${{ matrix.os == 'windows-latest' && '--filter="!./packages/vitest-pool-workers"' || '' }}
         env:
           NODE_OPTIONS: "--max_old_space_size=8192"
           WRANGLER_LOG_PATH: ${{ runner.temp }}/wrangler-debug-logs/