Use flatware to parallelize CI specs (#30284)

spec/flatware_helper.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+if defined?(Flatware)
+  Flatware.configure do |config|
+    config.after_fork do |test_env_number|
+      unless ENV.fetch('DISABLE_SIMPLECOV', nil) == 'true'
+        require 'simplecov'
+        SimpleCov.at_fork.call(test_env_number) # Combines parallel coverage results
+      end
+    end
+  end
+end