test_thread.rb: let main thread exit from Thread.new

* test/ruby/test_thread.rb (test_main_thread_status_at_exit): let
  the main thread run to ensure exiting from Thread.new.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47224 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-08-20

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47225 b2dd03c8-39d4-4d8f-98ff-823fe69b080e