diff --git a/swh/journal/tests/utils.py b/swh/journal/tests/utils.py
--- a/swh/journal/tests/utils.py
+++ b/swh/journal/tests/utils.py
@@ -37,6 +37,12 @@
 
 
 class MockedKafkaConsumer:
+    """Mimic the confluent_kafka.Consumer API, producing the messages stored
+       in `queue`.
+
+       You're only allowed to subscribe to topics in which the queue has
+       messages.
+    """
     def __init__(self, queue):
         self.queue = queue
         self.committed = False
@@ -48,6 +54,14 @@
         if self.queue == []:
             self.committed = True
 
+    def list_topics(self, timeout=None):
+        return set(message.topic() for message in self.queue)
+
+    def subscribe(self, topics):
+        unknown_topics = set(topics) - self.list_topics()
+        if unknown_topics:
+            raise ValueError('Unknown topics %s' % ', '.join(unknown_topics))
+
 
 class MockedJournalClient(JournalClient):
     def __init__(self, queue, object_types=ACCEPTED_OBJECT_TYPES):