Don't fail the autopkgtest job if all tests are skipped or superficial.

autopkgtest returns exit code 8 if there no tests in the package, or if all non-superficial tests were skipped, or if all tests pass but are marked as "superficial" (for details about the "superficial" test restriction, see https://bugs.debian.org/904979). Whenever this happens, on the current Debian infrastructure the test results won't affect migration-to-testing delay: they won't add delay as a failure would, but they won't decrease the delay either as successful non-superficial tests would.

Assuming there's no way to express this in GitLab CI (AFAIK a test can only succeed or fail), in this situation it's better to succeed than to fail: if we always consider such skipped/superficial test suite runs as failed, the package maintainer will simply get used to seeing them in failed state and will not have any easy mean to notice if a regression makes the autopkgtests actually fail… which kinda defeats the point of running them in the first place.