autopkgtest: Handle status '8' as a pass

This means 'all tests were skipped', or 'no tests in this package'. The
former case is new after our recent autopkgtest rebase.

Upstream britney (624b185ba6) handles this
as "neutral", which doesn't influence the result and is a more explicit
(better) way of expressing this situation. Once we rebase, we should
take that approach.

britney2/policies/autopkgtest.py

@@ -554,7 +554,7 @@ class AutopkgtestPolicy(BasePolicy):
 
         stamp = os.path.basename(os.path.dirname(url))
         # allow some skipped tests, but nothing else
-        passed = exitcode in [0, 2]
+        passed = exitcode in [0, 2, 8]
 
         self.log('Fetched test result for %s/%s/%s %s (triggers: %s): %s' % (
             src, ver, arch, stamp, result_triggers, passed and 'pass' or 'fail'))