Fix an intermittent spec failure

What does this MR do?

Fixes an intermittent spec failure in spec/features/projects/members/master_adds_member_with_expiration_date_spec.rb detected during the CE->EE merge: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2422#note_34945597

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

Why was this MR needed?

Intermittent spec failures are bad

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together