Configure Active Storage on dummy apps for extensions

Copying the .yml configuration file into config/storage/test.yml
will automatically configure Active Storage in that env [1].

The configuration key needs to be the same of the service
set into config/environments/test.rb.

When testing extensions against old versions of Solidus, it
will not use Active Storage so, it should not be harmful to
have it configured anyway.

[1]: https://github.com/rails/rails/blob/66b861f24cd8ca34b726f77b46f6b25b21f0b972/activestorage/lib/active_storage/engine.rb#L115-L132

core/lib/generators/spree/dummy/dummy_generator.rb

@@ -55,6 +55,7 @@ def test_dummy_config
       @database = options[:database]
 
       template "rails/database.yml", "#{dummy_path}/config/database.yml", force: true
+      template "rails/storage.yml", "#{dummy_path}/config/storage/test.yml", force: true
       template "rails/boot.rb", "#{dummy_path}/config/boot.rb", force: true
       template "rails/application.rb.tt", "#{dummy_path}/config/application.rb", force: true
       template "rails/routes.rb", "#{dummy_path}/config/routes.rb", force: true

core/lib/generators/spree/dummy/templates/rails/storage.yml

@@ -0,0 +1,3 @@
+test:
+  service: Disk
+  root: <%= File.expand_path("#{dummy_path}/tmp/storage", destination_root) %>

core/lib/generators/spree/dummy/templates/rails/test.rb

@@ -29,6 +29,8 @@
   config.action_mailer.delivery_method = :test
   ActionMailer::Base.default from: "solidus@example.com"
 
+  config.active_storage.service = :test
+
   # Raise on deprecation warnings
   if ENV['SOLIDUS_RAISE_DEPRECATIONS'].present?
     Spree::Deprecation.behavior = :raise