_message.py: Assert elapsed is set for SKIPPED

buildstream/_message.py

@@ -76,7 +76,7 @@ class Message():
         self.task_id = task_id            # The plugin object ID of the task
         self.scheduler = scheduler        # Whether this is a scheduler level message
         self.creation_time = datetime.datetime.now()
-        if message_type in (MessageType.SUCCESS, MessageType.FAIL):
+        if message_type in (MessageType.SUCCESS, MessageType.FAIL, MessageType.SKIPPED):
             assert elapsed is not None
         # logfile should always be associated with these messages
         if message_type in (MessageType.START, MessageType.SUCCESS, MessageType.FAIL, MessageType.SKIPPED):