autopkgtests accidentally in dry-run mode everywhere, fix logic

britney.py

@@ -523,7 +523,7 @@ class Britney(object):
             self._policy_engine.add_policy(PiupartsPolicy(self.options, self.suite_info))
         add_autopkgtest_policy = getattr(self.options, 'adt_enable', 'no')
         if add_autopkgtest_policy in ('yes', 'dry-run'):
-            self._policy_engine.add_policy(AutopkgtestPolicy(self.options, self.suite_info, dry_run=add_autopkgtest_policy))
+            self._policy_engine.add_policy(AutopkgtestPolicy(self.options, self.suite_info, dry_run=add_autopkgtest_policy == 'dry-run'))
         self._policy_engine.add_policy(AgePolicy(self.options, self.suite_info, MINDAYS))
         # XXX this policy results in asymmetric enforcement of
         # build-dependencies in the release pocket (nothing blocks