Fix a spec failure dependent on load order

We were incorrectly stubbing `Issues::MoveService#execute`. This only
caused a failure when another spec caused it to be autoloaded --
`spec/controllers/projects/issues_controller_spec.rb` in this case.

spec/services/issues/update_service_spec.rb

@@ -519,7 +519,7 @@ describe Issues::UpdateService, :mailer do
         end
 
         it 'calls the move service with the proper issue and project' do
-          move_stub = class_double("Issues::MoveService").as_stubbed_const
+          move_stub = instance_double(Issues::MoveService)
           allow(Issues::MoveService).to receive(:new).and_return(move_stub)
           allow(move_stub).to receive(:execute).with(issue, target_project).and_return(issue)