GitLab pages: Improve test pages_domain_spec.rb

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What does this MR do and why?

• Related to #467235
• • During the work on this MR, I identified possible improvements for the readability of the test code; extracted the changes in this MR to keep it focussed.
• Apply table syntax where block for hostname context

🛠 with ❤ at Siemens

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

MR Checklist (@gerardo-navarro)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Screenshots or screen recordings

Only changes related to test

How to set up and validate locally

1. Execute test

bundle exec bin/rspec ./spec/models/pages_domain_spec.rb