Improve snippet model spec

What does this MR do?

In this MR we improve the spec spec/models/snippet_spec.rb.

In local, the time before the changes is 30 seconds. The current time is 17 seconds. Therefore, the improvement is 13 seconds.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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Closes #228708 (closed)