Add a test for Gitlab::Seeder.parallel_each

What does this MR do and why?

This adds a test for the Gitlab::Seeder.parallel_each function, which is used by seeders to parallelize insert opertation across threads.

When we added this helper function, we didn't add specs for it. This ensures that the function works as expected.

References

• Gitlab::Seeder.parallel_each introduced in !205559 (merged)

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