CE port of "qa-staging-15-use-existing-users-ee"

What does this MR do?

This is CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8266

Makes use of existing users in the test since new users cannot be created in staging.

What are the relevant issue numbers?

gitlab-org/quality/staging#15 (closed)

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.