tests: Extend producer example test with additional checks

This patch extends the `test_examples.py` integration test for the
simple producer example with two additional checks inspired by the new
producer-with-correlation example test.  First, we make sure that the
producer process completes successfully.  Second, we verify that it only
sent one message to the consumer while it was alive.

Signed-off-by: Patrick M. Niedzielski <[email protected]>

tests/integration/test_examples.py

@@ -82,6 +82,8 @@ def test_example_producer(unique_queue):
 
     # THEN
     session.stop()
+    process.wait()
+    assert q.empty()
     assert msg.data == b"\xde\xad\x00\x00\xbe\xef"
     assert msg.queue_uri == unique_queue
     assert msg.properties == {}