Adjust tests to api admin mode (1)

What does this MR do and why?

Adjusts all tests from /spec/requests/api/admin/ for api admin mode.

Note: also added one test from /spec/frontend/fixtures as it was the only one in this directory.

Adjusted shared_examples from !111198 (merged) so that the success code for the 'DELETE' http method can be set dynamicly. (needed in spec/requests/api/admin/sidekiq_spec.rb)

issue

implementation plan

This MR is a follow up of !108690 (merged) and !111198 (merged)

🛠 with ❤ at Siemens

How to set up and validate locally

1. Run bin/rspec spec/requests/api/admin/batched_background_migrations_spec.rb
2. Run bin/rspec spec/requests/api/admin/ci/variables_spec.rb
3. Run bin/rspec spec/requests/api/admin/instance_clusters_spec.rb
4. Run bin/rspec spec/requests/api/admin/plan_limits_spec.rb
5. Run bin/rspec /spec/requests/api/admin/sidekiq_spec.rb
6. Run bin/rspec spec/frontend/fixtures/api_deploy_keys.rb

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.