Parallelize tests in the CI

Author: santib

railties/lib/rails/generators/rails/app/templates/github/ci.yml.tt

@@ -93,6 +93,9 @@ jobs:
           ruby-version: .ruby-version
           bundler-cache: true
 
+      - name: Configure parallelization
+        run: echo "PARALLEL_WORKERS=$(nproc)" >> $GITHUB_ENV
+
       - name: Run tests
         env:
           RAILS_ENV: test