ci: let GitHub Actions upload failed tests' directories

Arguably, CI builds' most important task is to not only identify
regressions, but to make it as easy as possible to investigate what went
wrong.

In that light, we will want to provide users with a way to inspect the
tests' output as well as the corresponding directories.

This commit adds build steps that are only executed when tests failed,
uploading the relevant information as build artifacts. These artifacts
can then be downloaded by interested parties to diagnose the failures
more efficiently.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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