ci: Upload test suite logs when tests fail

We sometimes see unreproducible test failures in CI. These are often hard to debug because they don't occur all the time, and may even be triggered by something that only happens on a specific CI runner.

As a step towards debugging these, we should upload the test log as a GitLab artifact whenever we see a CI failure. Meson only shows the last 100 lines of the test log on stdout, and there seems to be no way to cause it to emit more info, so this should help with debugging #79 (closed).