Improve test clean up for migration spec

What does this MR do and why?

This improves clean up for spec/migrations/20230809104753_swap_epic_user_mentions_note_id_to_bigint_for_self_hosts_spec.rb to fix a master:broken issue - gitlab-org/quality/engineering-productivity/master-broken-incidents#3341 (closed).

This is same as what we have fixed with - !127303 (merged).

We also update spec/migrations/20230810103534_swap_suggestions_note_id_to_bigint_for_self_hosts_spec.rb as it has the same problem.

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