Move TestServer and TestWorker under global namespace
Move TestServer and TestWorker under global namespace

This change can suppress following warnings(84 lines):

spec/server_worker_context.rb:48: warning: method redefined; discarding old initialize
spec/server_worker_context.rb:48: warning: previous definition of initialize was here
spec/server_worker_context.rb:52: warning: method redefined; discarding old before_run
spec/server_worker_context.rb:52: warning: previous definition of before_run was here
spec/server_worker_context.rb:56: warning: method redefined; discarding old after_run
spec/server_worker_context.rb:56: warning: previous definition of after_run was here
spec/server_worker_context.rb:60: warning: method redefined; discarding old after_start
spec/server_worker_context.rb:60: warning: previous definition of after_start was here
spec/server_worker_context.rb:64: warning: method redefined; discarding old stop
spec/server_worker_context.rb:64: warning: previous definition of stop was here
spec/server_worker_context.rb:74: warning: method redefined; discarding old restart
spec/server_worker_context.rb:74: warning: previous definition of restart was here
spec/server_worker_context.rb:84: warning: method redefined; discarding old reload
spec/server_worker_context.rb:84: warning: previous definition of reload was here
spec/server_worker_context.rb:89: warning: method redefined; discarding old detach
spec/server_worker_context.rb:89: warning: previous definition of detach was here
spec/server_worker_context.rb:96: warning: method redefined; discarding old initialize
spec/server_worker_context.rb:96: warning: previous definition of initialize was here
spec/server_worker_context.rb:101: warning: method redefined; discarding old before_fork
spec/server_worker_context.rb:101: warning: previous definition of before_fork was here
spec/server_worker_context.rb:105: warning: method redefined; discarding old run
spec/server_worker_context.rb:105: warning: previous definition of run was here
spec/server_worker_context.rb:115: warning: method redefined; discarding old stop
spec/server_worker_context.rb:115: warning: previous definition of stop was here
spec/server_worker_context.rb:120: warning: method redefined; discarding old reload
spec/server_worker_context.rb:120: warning: previous definition of reload was here
spec/server_worker_context.rb:124: warning: method redefined; discarding old after_start
spec/server_worker_context.rb:124: warning: previous definition of after_start was here

fluent/serverengine