Use SKIPPED result for autopkgtest runs that executed 0 tests

debusine marks autopkgtests as "Success" even when they merely Skipped the available test:
https://debusine.freexian.com/freexian/elts-staging/work-request/117204/
https://debusine.freexian.com/freexian/elts-staging/work-request/117229/
#1273 (closed)

I suspect there's a similar issue with Superficial tests.

As a package Uploader, unless I check each and every autopkgtest task individually (which I only did above because they was discrepancy with the ELTS britney2 instance), I won't be aware that no test was actually done.

So maybe we need more granularity in the tasks result, or a different debian_pipeline output template, to quickly determine if the tests were actually successful and relevant.