Fix building assets in test environment

Webpacker (Ruby wrapper to webpack) uses RAILS_ENV-based environments while
the javascript configuration for webpack re-reads this configuration file using
the NODE_ENV environment variable. This means that when RAILS_ENV=test, running
“assets:precompile” compiled the production packs in “public/packs” while
webpacker expects them in “public/packs-test”. This causes Ruby to recompile
them on-the-fly, possibly leading to race conditions in parallel_tests.

This changes:
- Disables on-the-fly compilation in test environment
- Changes the javascript part to read the correct environment

config/webpacker.yml

@@ -28,6 +28,10 @@ test:
   # Compile test packs to a separate directory
   public_output_path: packs-test
 
+  # CircleCI precompiles packs prior to running the tests.
+  # Also avoids race conditions in parallel_tests.
+  compile: false
+
 production:
   <<: *default