CI: Eager Load classes before tests

This is the Rails default.

From the docs

> Eager loading loads your whole application. When running a single test locally,
> this probably isn't necessary. It's a good idea to do in a continuous integration
> system, or in some way before deploying your code.

core/lib/spree/testing_support/dummy_app.rb

@@ -84,7 +84,7 @@ class Application < ::Rails::Application
     config.log_level = :debug
 
     # Improve test suite performance:
-    config.eager_load = false
+    config.eager_load = ENV['CI'].present?
     config.public_file_server.headers = { 'Cache-Control' => 'public, max-age=3600' }
     config.cache_store = :memory_store