MergeRequests::UpdateService and MergeRequest::RefreshService specs have severe technical debt

As pointed in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7328#note_101411393 :

After working on this MR I noticed that both the MergeRequests::UpdateService and MergeRequests::RefreshService specs could use a much needed refactor namely:

• Not use instance variables
• Separate EE-only specs to reside under the ee/ folder