Improve GraphQL performance specs for alert management

What does this MR do?

This improves the alert_management/alert performance specs by separating out the query validation from the N+1 test.

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.