WIP: Document Promise testing best practice
What does this MR do?
Add a section to the frontend guidelines about writing tests with Promises.
(same as https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11284)
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Add a section to the frontend guidelines about writing tests with Promises.
(same as https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11284)