gitlab-ci: upload Meson test logs as JUnit reports

When running tests, Meson knows to output both a test log as well as a
JUnit test report that collates results. We don't currently upload these
results in our GitLab CI at all, which makes it hard to see which tests
ran, but also which of our tests may have failed.

Upload these JUnit reports as artifacts to make this information more
accessible. Note that we also do this for some jobs that don't use Meson
and thus don't generate these reports in the first place. GitLab CI
handles missing reports gracefully though, so there is no reason to
special-case those jobs that don't use Meson.

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