Build assets automatically if necessary

In CI, these are unnecessary because done by preprocessing.
To test locally, you need to do it manually. If assets not exist, it will
fail when a run specific test, so force to build those only when necessary.

railties/test/isolation/abstract_unit.rb

@@ -488,7 +488,14 @@ class ActiveSupport::TestCase
     f.puts "require 'rails/all'"
   end
 
+  unless File.exist?("#{RAILS_FRAMEWORK_ROOT}/actionview/lib/assets/compiled/rails-ujs.js")
+    Dir.chdir("#{RAILS_FRAMEWORK_ROOT}/actionview") { `yarn build` }
+  end
+
   assets_path = "#{RAILS_FRAMEWORK_ROOT}/railties/test/isolation/assets"
+  unless Dir.exist?("#{assets_path}/node_modules")
+    Dir.chdir(assets_path) { `yarn install` }
+  end
   FileUtils.cp("#{assets_path}/package.json", "#{app_template_path}/package.json")
   FileUtils.cp("#{assets_path}/config/webpacker.yml", "#{app_template_path}/config/webpacker.yml")
   FileUtils.cp_r("#{assets_path}/config/webpack", "#{app_template_path}/config/webpack")