Flaky test in spec/workers/merge_requests/assignees_change_worker_spec.rb

As seen in https://gitlab.com/gitlab-org/gitlab/-/jobs/1160351743:

  1) MergeRequests::AssigneesChangeWorker#perform gets MergeRequests::UpdateAssigneesService to handle the changes
     Failure/Error: service.handle_assignee_changes(merge_request, users)
       #<MergeRequests::UpdateAssigneesService> received :handle_assignee_changes with unexpected arguments
         expected: (#<MergeRequest id:302 namespace1618/project1708!1>, [#<User id:2720 @user2777>, #<User id:2721 @user2778>, #<User id:2722 @user2779>])
              got: (#<MergeRequest id:302 namespace1618/project1708!1>, #<ActiveRecord::Relation [#<User id:2721 @user2778>, #<User id:2720 @user2777>, #<User id:2722 @user2779>]>)
       Diff:
       @@ -1,5 +1,3 @@
        [#<MergeRequest id:302 namespace1618/project1708!1>,
       - [#<User id:2720 @user2777>,
       -  #<User id:2721 @user2778>,
       -  #<User id:2722 @user2779>]]
       + #<ActiveRecord::Relation [#<User id:2721 @user2778>, #<User id:2720 @user2777>, #<User id:2722 @user2779>]>]
     # ./app/workers/merge_requests/assignees_change_worker.rb:26:in `perform'
     # ./spec/workers/merge_requests/assignees_change_worker_spec.rb:56:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:357:in `block (3 levels) in <top (required)>'
     # ./spec/support/sidekiq_middleware.rb:9:in `with_sidekiq_server_middleware'
     # ./spec/spec_helper.rb:348:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:344:in `block (3 levels) in <top (required)>'
     # ./lib/gitlab/application_context.rb:31:in `with_raw_context'
     # ./spec/spec_helper.rb:344:in `block (2 levels) in <top (required)>'
Finished in 16 minutes 8 seconds (files took 53.15 seconds to load)
4464 examples, 1 failure, 13 pending
Failed examples:
rspec ./spec/workers/merge_requests/assignees_change_worker_spec.rb:52 # MergeRequests::AssigneesChangeWorker#perform gets MergeRequests::UpdateAssigneesService to handle the changes