Fix test that check mentions for XSS

What does this MR do?

By simply tagging @eve the comment does not turn into a link, while by tagging @#{user.username} it turns into a link that when clicked could lead to an XSS attack, and this is what made me change the test.

Screenshots

Before

After

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