Revert "Revert "Proxy webpack dev server in test environment""

This restores the changes in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79457 by
reverting a2ba70ec.

The root cause should have be solved by the upgrade to rack-proxy
(https://gitlab.com/gitlab-org/gitlab/-/merge_requests/83241). This
upgrade should prevent chunked transfers from being proxied through
Workhorse.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/352649

app/helpers/webpack_helper.rb

@@ -83,16 +83,8 @@ module WebpackHelper
   end
 
   def webpack_public_host
-    # We do not proxy the webpack output in the 'test' environment,
-    # so we must reference the webpack dev server directly.
-    if Rails.env.test? && Gitlab.config.webpack.dev_server.enabled
-      host = Gitlab.config.webpack.dev_server.host
-      port = Gitlab.config.webpack.dev_server.port
-      protocol = Gitlab.config.webpack.dev_server.https ? 'https' : 'http'
-      "#{protocol}://#{host}:#{port}"
-    else
-      ActionController::Base.asset_host.try(:chomp, '/')
-    end
+    # We proxy webpack output in 'test' and 'dev' environment, so we can just use asset_host
+    ActionController::Base.asset_host.try(:chomp, '/')
   end
 
   def webpack_public_path

config/initializers/static_files.rb

@@ -21,7 +21,7 @@ if app.config.public_file_server.enabled
 
   # If webpack-dev-server is configured, proxy webpack's public directory
   # instead of looking for static assets
-  if Gitlab.config.webpack.dev_server.enabled && Rails.env.development?
+  if Gitlab.config.webpack.dev_server.enabled && Gitlab.dev_or_test_env?
     app.config.middleware.insert_before(
       Gitlab::Middleware::Static,
       Gitlab::Webpack::DevServerMiddleware,