Fix flaky transfer project spec

What does this MR do and why?

Assert that the transfer request has actually completed before querying the DB and asserting on backend data.

The spec is flaky because wait_for_requests is unreliable. See https://gitlab.com/gitlab-org/gitlab/-/work_items/595020#note_3259564899 for context.

Example failure: https://gitlab.com/gitlab-org/gitlab/-/jobs/14665423756

References

#525094 (closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.