test_flip.rb: flip-flop shared by threads

* test/ruby/test_flip.rb (TestFlip#test_shared_thread): flip-flop
  should be separated per threads

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38747 b2dd03c8-39d4-4d8f-98ff-823fe69b080e