fix(TestStreamClient): remove unused/unclosed mock producer (#74)

TestStreamClient sets the producer_class to start the mock producer on startup. If a mock producer is created beforehand it is immediately overwritten and leaves an unclosed producer, raising error logs from aiokafka.

kstreams/test_utils/test_utils.py

@@ -29,12 +29,7 @@ def mock_streams(self) -> None:
         for stream in streams:
             stream.consumer_class = TestConsumer
 
-    def mock_producer(self) -> None:
-        producer = TestProducer()
-        self.stream_engine._producer = producer
-
     def setup_mocks(self) -> None:
-        self.mock_producer()
         self.mock_streams()
 
     async def start(self) -> None: