Fix jobs_request_post_spec on sharding POC

What does this MR do?

Re-enables a commented spec.

The error was due to Ci::Group existing, and:

NoMethodError (undefined method `where' for Ci::Group:Class
Did you mean?  when):
  /Users/ak/Work/Gitlab/gitlab-development-kit/gitlab/app/services/ci/queue/build_queue_service.rb:30:in `builds_for_group_runner'
  /Users/ak/Work/Gitlab/gitlab-development-kit/gitlab/app/services/ci/register_job_service.rb:112:in `each_build'
  /Users/ak/Work/Gitlab/gitlab-development-kit/gitlab/app/services/ci/register_job_service.rb:54:in `process_queue'
  /Users/ak/Work/Gitlab/gitlab-development-kit/gitlab/app/services/ci/register_job_service.rb:30:in `block in execute'
  /Users/ak/Work/Gitlab/gitlab-development-kit/gitlab/lib/gitlab/ci/queue/metrics.rb:106:in `observe_queue_time'
  /Users/ak/Work/Gitlab/gitlab-development-kit/gitlab/app/services/ci/register_job_service.rb:29:in `execute'
  /Users/ak/Work/Gitlab/gitlab-development-kit/gitlab/lib/api/ci/runner.rb:146:in `block (2 levels) in <class:Runner>'

See #336555 (closed)

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