Add transient bug test docs
Documents how to run tests for transient bugs.
See https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/762 and https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/761
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Documents how to run tests for transient bugs.
See https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/762 and https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/761