test: build and verify addon runtime

stelloauth/rootfs/usr/local/bin/addon-supervisor

@@ -31,6 +31,7 @@ CLOAK_COMMAND = [
     "--data-dir=/tmp/cloakserve",
 ]
 STELLOAUTH_COMMAND = ["/usr/local/bin/stelloauth"]
+SHUTDOWN_GRACE_SECONDS = 9.0
 
 
 class ConfigError(RuntimeError):
@@ -297,7 +298,7 @@ class ProcessManager:
 
     def _begin_shutdown(self) -> None:
         if self._shutdown_deadline is None:
-            self._shutdown_deadline = self._monotonic() + 10.0
+            self._shutdown_deadline = self._monotonic() + SHUTDOWN_GRACE_SECONDS
         if not self._term_sent:
             self._signal_groups(signal.SIGTERM)
             self._term_sent = True