specs: precompile Webpack assets before running the specs suite

This avoids the first feature spec stalling for a few dozens of seconds
because Webpack assets are compiling (and thus reduce the risk of the
spec timing out).

Pre-compilation takes ~ 10s.

.circleci/config.yml

@@ -69,6 +69,11 @@ jobs:
             DATABASE_URL: "postgres://tps_test@localhost:5432/tps_test"
           name: Create DB
           command: bundle exec rake db:create db:schema:load db:migrate RAILS_ENV=test
+      - run:
+          environment:
+            RAILS_ENV: test
+          name: Precompile Webpack assets
+          command: bin/webpack
       - run:
           environment:
             DATABASE_URL: "postgres://tps_test@localhost:5432/tps_test"