ci: split out logic to set up failed test artifacts

ci: split out logic to set up failed test artifacts

We have some logic in place to create a directory with the output from
failed tests, which will then subsequently be uploaded as CI artifacts.
We're about to add support for GitLab CI, which will want to reuse the
logic.

Split the logic into a separate function so that it is reusable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>