Allow parallel testing (#35030).

Patch by Go MAEDA.


git-svn-id: http://svn.redmine.org/redmine/trunk@20920 e93f8b46-1217-0410-a6f0-8f06a7374b81

doc/RUNNING_TESTS

 You can run `ruby test/unit/issue_test.rb` for running a single test case and
 `ruby test/unit/issue_test.rb -n test_create` for running a single test.
 
+You can run tests in parallel by setting the PARALLEL_WORKERS environment
+variable:
+`PARALLEL_WORKERS=8 rake test`
+
 Before running tests, you need to configure both development
 and test databases.
 

test/test_helper.rb

 end
 
 class ActiveSupport::TestCase
+  parallelize(workers: 1)
+
   include ActionDispatch::TestProcess
 
   self.use_transactional_tests = true