Add a DummyApp sprockets 4 configuration manifest

This is needed from Sprockets, since v4. It contains
all dependecies that needs to be compiled.

Author: kennyadsl

core/lib/spree/testing_support/dummy_app.rb

@@ -123,6 +123,7 @@ class Application < ::Rails::Application
     ActiveRecord::Migrator.migrations_paths = migration_dirs
     ActiveRecord::Migration.verbose = false
 
+    config.assets.config_manifest = File.expand_path('dummy_app/assets/config/manifest.js', __dir__)
     config.assets.paths << File.expand_path('dummy_app/assets/javascripts', __dir__)
     config.assets.paths << File.expand_path('dummy_app/assets/stylesheets', __dir__)
     config.assets.css_compressor = nil

core/lib/spree/testing_support/dummy_app/assets/config/manifest.js

@@ -0,0 +1,3 @@
+// This manifest is left empty because we are already
+// injecting Solidus manifests to be compiled in the
+// precompile array manually.