The problem was that in wait_for_message_count_in_log function from srv_msg.py there is a line:
log_file, count, line = test_define_value(log_file, count, line)This is converting log_file that is None to "None". This blows up further code that expected just None and in result it produced: "/usr/local/var/log/None" instead of "/usr/local/var/log/kea.log".