Avoid busy retry test_consumer_group.py

Test test/test_consumer_group.py::test_group busy-retry and this might
have caused Java not having enough CPU time on GitHub runner, and result
in test failure.

test/test_consumer_group.py

@@ -111,6 +111,7 @@ def consumer_thread(i):
                     logging.info('Rejoining: %s, generations: %s', rejoining, generations)
                     time.sleep(1)
             assert time.time() < timeout, "timeout waiting for assignments"
+            time.sleep(0.25)
 
         logging.info('Group stabilized; verifying assignment')
         group_assignment = set()