Change CircleCI config to build assets with NODE_ENV=test

This should avoid minimization and other expensive webpack
transformations which aren't useful for running tests.

.circleci/config.yml

@@ -9,6 +9,7 @@ aliases:
           DB_HOST: localhost
           DB_USER: root
           RAILS_ENV: test
+          NODE_ENV: test
           PARALLEL_TEST_PROCESSORS: 4
           ALLOW_NOPAM: true
           CONTINUOUS_INTEGRATION: true