Load an application before use

Without this, `Rails.application` returns `nil`.
Ref: https://buildkite.com/rails/rails/builds/65683#e86ec58e-53a5-41be-8aeb-11c5705ee580/1042-1053

railties/test/application/configuration_test.rb

@@ -2498,6 +2498,8 @@ module ApplicationTests
         config.rake_eager_load = true
       RUBY
 
+      app "development"
+
       assert_equal true, Rails.application.config.rake_eager_load
     end