Remove `Sidekiq::Testing.fake!`

What does this MR do?

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/22992

Are there points in the code the reviewer needs to double check?

Its not possible to use perform_enqueued_jobs because the gitlab workers does not use ActiveJob API, gitlab sidekiq workers have to first migrate to ActiveJob, cf https://github.com/mperham/sidekiq/wiki/Active-Job

Why was this MR needed?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/22992

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/22992