Now we need to set dev_server config to Rails.env.test

```
Error:
PagesControllerTest#test_it_mounts_components_from_the_dev_server:
RuntimeError: Failed to start dev server
    /home/travis/build/reactjs/react-rails/test/support/webpacker_helpers.rb:65:in `with_dev_server'
    /home/travis/build/reactjs/react-rails/test/react/rails/pages_controller_test.rb:32:in `block (2 levels) in <class:PagesControllerTest>'
```

Related to rails/webpacker#1179

Author: ttanimichi

test/dummy_webpacker3/config/webpacker.yml

@@ -43,6 +43,12 @@ test:
   <<: *default
   compile: true
 
+  dev_server:
+    host: localhost
+    port: 8080
+    hmr: false
+    https: false
+
 production:
   <<: *default