Restore the sequence of test steps to unblock deploy

What does this MR do?

Issue: #215490 (closed)

Restore the sequence of test steps in check_mentions_for_xss_spec.rb and code_owners_spec.rb. It appeared after 5882b387 -> as Mark L. mentions the problem seems to be that gitlab-qa is logged in when the test tries to create a user, which fails because admin access is needed to create a user via the API. Possible fix is to restore the original sequence of the steps.

This MR restores the original code to unblock the deploy process until that. Tests pass successfully when using the code before 5882b387 was merged, so the issue is not with the application code.

Update: the first commit was to quarantine the specs, but decided to restore the original code instead, so we don't add to the quarantine backlog.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content