Fix invalid reference in linked work item spec

What does this MR do and why?

In feature specs, Gitlab.config.gitlab.url is different from the actual URL which includes a random port number. We need to pass in a reference that uses the configured URL because our reference parser depends on this to parse references.

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Screenshots or screen recordings

Sample failure screenshot:

In specs, Gitlab.config.gitlab.url (from gitlab.yml) is set to http://127.0.0.1.

How to set up and validate locally

bin/rspec spec/features/projects/work_items/linked_work_items_spec.rb:92

Related to #438014