Fix specs failing on duplicate gpg signature

What does this MR do?

It seems there are additional async requests related to gpg signature, w/o waiting for finishing all requests it fails on: Duplicate entry '...' for key 'index_gpg_signatures_on_commit_sha' in rails 5 environment:

   PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_gpg_signatures_on_commit_sha"
       DETAIL:  Key (commit_sha)=(\x33f3729a45c02fc67d00adb1b8bca394b0e761d9) already exists.
       : INSERT INTO "gpg_signatures" ("created_at", "updated_at", "project_id", "commit_sha", "gpg_key_primary_keyid", "verification_status") VALUES ('2018-08-12 07:15:03.035979', '2018-08-12 07:15:03.035979', 104, '\x33f3729a45c02fc67d00adb1b8bca394b0e761d9', '\x627c5f589f467f17', 5) RETURNING "id"

What are the relevant issue numbers?

Closes #50224 (closed)

Related to !21119 (merged) Related to !20987 (merged)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides