Ensure each QA run saves screenshots to a unique folder

This is the first part of https://gitlab.com/gitlab-org/gitlab-ce/issues/49021 which ensures subsequent QA runs don't erase screenshots from previous runs.

Part 2 (keep the test file structure for screenshots) is at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20495.

Artifacts structure locally (with the change from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20495)

Artifacts structure on the CI (without the change from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20495)

/cc @ddavison