esphome/tests/components/mqtt/test.esp32-idf.yaml

wifi:
  ssid: MySSID
  password: password1

mqtt:
  broker: test.mosquitto.org
  port: 1883
  discovery: true
  discovery_prefix: homeassistant
  idf_send_async: false
  log_topic:
  on_message:
    topic: testing/sensor/testing_sensor/state
    qos: 0
    then:
      - logger.log: Mqtt Test