config.plugin_paths in specific environment configs fails

I tried to put this line in test.rb, but the plugins in vendor/testing_plugins failed to load.

config.plugin_paths += %W[#{RAILS_ROOT}/vendor/testing_plugins]

If I put this in environment.rb, the plugins in vendor/testing_plugins are added to the load path.

if RAILS_ENV == "test"
  # this doesn't work if it's in test.rb
  config.plugin_paths += %W[#{RAILS_ROOT}/vendor/testing_plugins]
end