Fix building assets in test environment (#8691)

Fix building assets in test environment (#8691)

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