Updates check_mentions_for_xss_spec to use admin token for creating the user

What does this MR do?

Updates check_mentions_for_xss_spec to use admin token for creating the user

Closes gitlab-org/quality/staging#65 (closed)

Does this MR meet the acceptance criteria?

Conformity

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• [-] Documentation created/updated or follow-up review issue created
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• Separation of EE specific content