Suggest eager load in CI in the testing guide

Author: fxn

guides/source/testing.md

@@ -1972,6 +1972,54 @@ class ChatRelayJobTest < ActiveJob::TestCase
 end
 ```
 
+Testing Eager Loading
+---------------------
+
+Normally, applications do not eager load in the `development` or `test` environments to speed things up. But they do in the `production` environment.
+
+If some file in the project cannot be loaded for whatever reason, you better detect it before deploying to production, right?
+
+### Continuous Integration
+
+If your project has CI in place, eager loading in CI is an easy way to ensure the application eager loads.
+
+CIs typically set some environment variable to indicate the test suite is running there. For example, it could be `CI`:
+
+```ruby
+# config/environments/test.rb
+config.eager_load = ENV["CI"].present?
+```
+
+Starting with Rails 7, newly generated applications are configured that way by default.
+
+### Bare Test Suites
+
+If your project does not have continuous integration, you can still eager load in the test suite by calling `Rails.application.eager_load!`:
+
+#### minitest
+
+```ruby
+require "test_helper"
+
+class ZeitwerkComplianceTest < ActiveSupport::TestCase
+  test "eager loads all files without errors" do
+    assert_nothing_raised { Rails.application.eager_load! }
+  end
+end
+```
+
+#### RSpec
+
+```ruby
+require "rails_helper"
+
+RSpec.describe "Zeitwerk compliance" do
+  it "eager loads all files without errors" do
+    expect { Rails.application.eager_load! }.not_to raise_error
+  end
+end
+```
+
 Additional Testing Resources
 ----------------------------