Refactor check mentions for xss test

What does this MR do?

Move test pre-conditions to before block and leave only the test steps and verifications in the it block. Also, remove an unnecessary variable that was being used only in one place, and instead, use the string directly where it's needed.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content