Fix failing test on uninitialized `Rails.config.assets.prefix`

This was introduced in [2684f17a] 💣

actionpack/test/abstract_unit.rb

@@ -151,6 +151,7 @@ def config
       config.assets_dir = public_dir
       config.javascripts_dir = "#{public_dir}/javascripts"
       config.stylesheets_dir = "#{public_dir}/stylesheets"
+      config.assets          = ActiveSupport::InheritableOptions.new({ :prefix => "assets" })
       config
     end
   end