ci: make test slicing consistent across Meson/Make

ci: make test slicing consistent across Meson/Make

In the preceding commit we have adjusted test slicing to be one-based
when using the "ci/run-test-slice.sh" script. But we also have an
equivalent script for Meson that is still zero-based, which is of course
inconsistent.

Adapt the script to be one-based, as well, and adapt the GitHub workflow
accordingly. Note that GitLab doesn't yet use the script, so it does not
need to be adapted. This will change in the next commit though.

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