Move feature specs under the projects folder

Context

Related to #393364 (closed).

Related to this master-broken incident: #391609 (closed). Specifically, see this comment:

Yeah, the sad news is that spec/features/work_items/work_item_spec.rb isn't under spec/features/projects.

It got me digging as why that wasn't the case.

What does this MR do?

Move work_item feature specs inside the projects features folder.

Reasons:

• The rails controller is under projects as well: https://gitlab.com/gitlab-org/gitlab/blob/master/app/controllers/projects/work_items_controller.rb
• The rails routes in the feature specs are namespaced under projects
• It would make the detect-tests job actually run those specs on predictive pipelines

MR acceptance checklist

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