autopkgtest: Support Ubuntu's run_id format

These have a hash appended.

We don't actually use the baseline retrying, which is where the ID
parsing is used, but we might as well handle this, not least so we don't
crash.

britney2/policies/autopkgtest.py

@@ -891,7 +891,8 @@ class AutopkgtestPolicy(BasePolicy):
             return
 
         run_id = os.path.basename(os.path.dirname(url))
-        seen = round(calendar.timegm(time.strptime(run_id, '%Y%m%d_%H%M%S@')))
+        # 20200101_000000 is 15 chars long
+        seen = round(calendar.timegm(time.strptime(run_id[0:15], '%Y%m%d_%H%M%S')))
         # allow some skipped tests, but nothing else
         if exitcode in [0, 2]:
             result = Result.PASS