Fix flakey time-sensitive user/assignee autocomplete dropdown

What does this MR do?

Fix flakey time-sensitive user/assignee autocomplete dropdown

Found while working on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6051, failing test: https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/74649299

Are there points in the code the reviewer needs to double check?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/38664

This previously worked because, it would assert the correct names in the full list and then wait for the list to go blank for the NOT assertion (list goes blank because we need at least 3 characters, https://gitlab.com/gitlab-org/gitlab-ce/issues/38664)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • Not a user-facing change
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?