Fix SimpleCov following issue

simplecov-ruby/simplecov#718
Rails6のパラレルテスト対策

test/system/drills_test.rb

@@ -45,5 +45,8 @@ def login_as_basic_member
     first(".problem-choice").click
     assert_no_text "中断して採点する"
     assert_text "採点する"
+    click_button "採点する"
+
+    # result
   end
 end

test/test_helper.rb

@@ -1,17 +1,34 @@
-ENV['RAILS_ENV'] ||= 'test'
+# [Qiita: SimpleCovとRails 6のMinitest](https://qiita.com/kazutosato/items/382afd90caf64b644e77)
+# https://github.com/simplecov-ruby/simplecov/issues/718
+
+if ENV['COVERAGE']
+  require 'simplecov'
 
-require 'simplecov'
-SimpleCov.start
+  SimpleCov.start 'rails'
+end
 
-require_relative "../config/environment"
-require "rails/test_help"
+ENV['RAILS_ENV'] ||= 'test'
+require_relative '../config/environment'
+require 'rails/test_help'
 
 class ActiveSupport::TestCase
-  ## Run tests in parallel with specified workers
-  # parallelize(workers: :number_of_processors)
+  # Run tests in parallel with specified workers
+  parallelize(workers: :number_of_processors)
+
+  if ENV['COVERAGE']
+    parallelize_setup do |worker|
+      SimpleCov.command_name "#{SimpleCov.command_name}-#{worker}"
+    end
+
+    parallelize_teardown do |worker|
+      SimpleCov.result
+    end
+  end
 
   # Setup all fixtures in test/fixtures/*.yml for all tests in alphabetical order.
   fixtures :all
 
+  # Add more helper methods to be used by all tests here...
+
   # include FactoryBot::Syntax::Methods
 end