Refactor verify_pages_domain_service specs

What does this MR do?

Refactor verify_pages_domain_service specs

Add specs for new/disabled pages domain Extract contexts from examples Use let instead of set since domain is being changing in each example

This is ~backstage for https://gitlab.com/gitlab-org/gitlab-ce/issues/44696 And I think it worth extracting into separate MR

Does this MR meet the acceptance criteria?

• 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 database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer