test/unit.rb: extract delete_worker
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36386 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test/unit.rb: extract delete_worker
worker from working set.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36386 b2dd03c8-39d4-4d8f-98ff-823fe69b080e